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FAIR-CT96-1913 SORGHUM: Environmental studies on sweet and fibre sorghum sustainable crops for biomass and energy |
Introduction
Sorghum is native of central-eastern Africa and belongs to the botanical family of Graminaceae, genus Sorghum species bicolor (L,) [Moench]. Although its tropical origin it is well adapted to other climates. The tropical origin determines, by the way, its climatic requirements. In fact, it is a short-day plant and requires 14 hours for floral initiation. Actually, differences among genotypes are shown, so that photosensitive and photoinsensitive crops are known. Base temperature for growth and development seems to be around 9-10 °C. However a temperature assuring a faster emergence and a better Sorghum seedling establishment is around 14-15 °C. Cardinal temperatures differ among cultivars, the optimum one ranges between 33-34 °C for some authors or 27-29 °C for others.
Sorghum is a C4 plant, with a high net assimilation rate even in water stress or high light intensity conditions. In terms of water requirements Sorghum shows an ET coefficient lower than maize, and its resistance to water stress can be accompanied by a quite good resistance to high humidity conditions, The crop shows a good adaptability to many types of soil, sandy or clay, even with low pH and is resistant to saline soils. Cultivated Sorghums are included in three subspecies: bicolor (S.bicolor subsp. bicolor), drummondii and verticilliflorum. Classifications of Sorghum are frequently produced in based on their use: types are recognised for grain, forage, sugar and biomass for energy and paper production. From the races bicolor, several types have been selected, among which are:
Sweet Sorghum, in particular, could be used for production of ethanol and its derivatives from the fermentation of sugars present in the stem juices. It may also supply by-products from bagasse, the remaining part of the stems after juice extraction, such as pyrolytic oils, quality fuels, pellets of carbon, synthesis gas and lignocellulosic materials. Sweet Sorghum and more likely fiber Sorghum can be used for production for paper. An interesting energetic application may be electricity production through combustion of total biomass.
Sweet Sorghum has been investigated as a source of sugar in Italy in the past and more recently has been considered, at European level, as a crop for biomass for energy and pulp for paper production. As far as fiber Sorghum is concerned, previous work has shown it to produce a good quality fiber in the stalks but a low yield of biomass. Recently in Europe renewed interest in this crop has resulted from the availability of hybrids 'grain Sorghum x broomcorn', that provide a greater yield, resulting from the high heterosys provided by this cross.
Objectives
The overall aim of the project is to study the environmental impacts of sweet and fiber Sorghum within cropping systems with particular reference to nitrogen balance, and to assess the quantity and quality of these crops in order to meet the needs of EU in terms of biomass for energy and pulp for paper. The specific objectives of this project are:
These objectives are based on and should extend results from previous programmes at National and European Union level from which agronomic results were obtained on productivity (between 15 and 40 t/ha), response to nitrogen and water, response to low temperature and resistance to lodging. In addition such studies resulted in setting up of productivity models, economic evaluations and resolution of some aspects of harvesting.
Activities
Productivity Network Field experiments were carried out by participants in Italy, Spain, France, Greece and Portugal, at latitudes from 37° 23' lat N (Enna - Italy) to 44°30' N ( Bologna - Italy). Altitudes ranged from 20 m asl. (Metaponto-Italy) to 595 m asl (Madrid-Spain). During the first year of experiment (summer-autumn 1997), one fiber Sorghum (HI28) and one sweet Sorghum (MN1500) variety were studied. In the second and third year various participants used either 2 varieties (1 fiber and 1 sweet Sorghum) or 5 sweet Sorghum and fiber Sorghum varieties. Traditional techniques used for pre-seeding soil management (ploughing, tillage, harrowing). Sowing dates ranged between the 19th of May and 17th of June in 1997, between 1st of May and 12th of June in 1998 and from 10th of April to 28th of May in 1999.
Crop rotation Field experiments, carried during the three years period 1997-99, gave nineteen crop rotations, including fiber and sweet Sorghum, soybean, wheat, rapeseed, faba bean, Jerusalem artichoke and maize, which were compared at five locations. Crop rotation was compared on the basis of wheat production and consideration of the period of time soil was covered by vegetation. For this purpose the ratio between number of days during which a crop covered the soil and the total number of days of the experiment was calculated and termed the Field Cover Index. In order to evaluate the effects of different crop rotations, in all locations, on grain yield, dry matter yield, etc. primary data was normalised over the yearly, with mean values calculated for each location and species.
Low input Field experiments, using low levels of input, were carried out at three locations in Italy over three years. Each year, at each location a split-plot experiment with four or three replications, was carried out. The main plot treatments were two input levels (normal and low input), sub-plot variations were two genotypes (fibre and sweet Sorghum) at two densities (10 or 20 plants per sq m). The genotypes of sweet and fibre Sorghum used were MN1500 and HI 32 during the first year, and Keller and H128 during the last two years.
Selection of genotypes for low N input and low temperature Typical characteristics investigated were maximum biomass yield and content of fermentable components or, for different uses fibre. For some uses sugar composition is also. A wide range of maturity classes is required in order to extend the harvest period to meet the requirements of the processing factories. For this reason, but also for agronomical reasons, the poor tolerance to low temperatures in the early stages of growth in temperate regions is a significant problem for most Sorghum genotypes that has to be overcome through selection. At the same time the innate ability of this crop to exploit conditions poor in water and soil fertility must be preserved or increased by developing genotypes that give an economically effective yield under limiting environments. This problem was approached as follows.
Genetic activity The genetic variability for cold tolerance and nitrogen use efficiency (NUE) in the first life stages, available in a variety collection and in segregant progeny from crosses, was investigated. Information was also gathered concerning the relationships existing between cold tolerance and other important crop characteristics. The results can be considered in four sections concerning:
The germination tests were carried out at the following seven temperatures: 4°, 6°, 8°, 10°, 12°, 20° and 25°C. This last temperature was taken as the control, since it has been reported in the literature as optimal for the germination of Sorghum. For each samples the radicle protrusion was scored daily until no further visible radicle was observed. At the end of the experiments, the final germination percentage and the mean time to germination were calculated. In order to validate the results of laboratory experiments, open field trials were carried out in the 2nd (1998) and 3d (1999) year.
In 1998, the following factors applied: five sowing dates (from March to July, according to the Partner), 10 genotypes among sweet, grain and fibre types, a seed osmo-conditioning (250 g/l of PEG for 2 days at 15°C) compared to an untreated control. The investigation was limited to three genotypes sown in mid May in the UK, whereas further six genotypes were assessed in Italy. In 1999, three partners were involved in this research, with three sowing dates, four genotypes and three seed -treatments (untreated control, PEG osmo-conditioning with 250 g/l of PEG x 2 days at 15'C, seed washing) and germination substrates (natural soil and sand) have been studied. After sowing, the number of emerged seedlings has been recorded weekly for up to five weeks from sowing. At harvest, fresh and dry biomass on the whole plot has been measured.
Nitrogen balance and nitrate leaching All the experimental sites were located in the south of Europe, where the thermal regime is more favourable to sweet and paper Sorghum cultivation. In Italy the trials were carried out in Bologna (Cadriano, in the southern edge of Po River Valley, 10 Km from Bologna, 44°30'N, 11°21'E, 32 m a.s.l.), in Bari (Rutigliano, 15 Km from Bari, 41° N), in Potenza in the area of Gaudiano di Lavello (PZ, 41°03' N, 15°41'E, 180 m a.s.l. and in Enna (in a hilly area of inner Sicily, Barrafranea, 550 m a.s.l., 37°23' N, 14°12 E). In Greece the experiment took place in the fertile plain of Kopais, 100 Km NW of Athens and in Spain in Madrid. The climate of all the experimental sites is semi-arid, with temperate and rainy winter and hot and dry summer. The work focused on the environmental risks linked to nitrogen fertilisation and its interaction with the irrigation regime. As a consequence of the summer growing cycle of this species, irrigation is necessary in most of the cropping areas for an adequate yield.
In this context, the interaction between the irrigation regime and nitrogen fertilisation is crucial due to the risks of nitrogen and nitrate leaching into the groundwater. A knowledge of the nitrogen balance can be also useful to detect the variations of the nitrogen soil reserves after the cultivation period, in order to define the sustainability of the soil use by the crop. The exact determination of the crop uptake is in fact the base for a rational fertilisation plan, while the dynamic of uptake is essential for the optimisation of the timing of fertilisation; it is also crucial for the modelling activity. The same terms involved in the nitrogen balance are also involved in the computation of the nitrogen use efficiency (N uptake efficiency, apparent N recovery, N fertilisation efficiency, N agronomic efficiency) and, as a consequence, are required in order to have indications about the effectiveness of nitrogen fertilisation and can provide information allowing for a more careful choice of the nitrogen fertiliser rate used. The terms of the nitrogen balance measured within the project were:
F = total nitrogen supplied by fertiliser;
R&IW = total nitrogen supplied by rainfall and irrigation water;
CU = total nitrogen uptake by the crop;
D = total nitrogen lost for volatilisation-,
L = total nitrogen lost for leaching and erosion
+/-deltaSR = variation in soil nitrogen reserve.
Water balance The main terms of the water balance were also measured to study the interaction with the irrigation regime. In addition to being highly productive in terms of biomass, sweet Sorghum is also known to show high drought and water logging resistance and salinity tolerance. For these reasons it is considered as the "camel" among the biomass-energy crops. In the Mediterranean regions, previous studies on sweet Sorghum have been conducted to assess its potential productivity and water requirement under non-limiting conditions. However, since water resources in the Mediterranean region are limited, the success of sweet Sorghum in this area depends upon the optimisation of water supplied by irrigation.
This study was carried out in four sites representative of Mediterranean region. In a first step the method for calculating the terms of the water balance equation was calibrated. The second step was to evaluated water requirements and water use efficiency, both in well watered soil condition and during water shortage. The aim of this task was to suggest an irrigation strategy for sweet and fibre Sorghum growing under Mediterranean environment. The effects of various water regimes on water consumption were studied during three seasons at different localities:
| Country | Region | Site | Latitude N | Longitude E | Altitude (m) |
| Italy | Puglia | Rutigliano | 41° 01 | 17° | 122 |
| Italy | Basilicata | Gaudiano | 41° 03 | 15° 03 | 350 |
| Italy | Sicily | Barrafranca | 550 | ||
| Greece | Attiki | Vagia |
The Sorghum hybrids used were:
| Site | 1997 | 1998 | 1999 | type |
| Puglia | MN1500 | Keller | Keller | sweet |
| Basilicata | MN1500 | Keller | Keller | sweet |
| Sicily | MN1500 | Keller | Keller | sweet |
| Sicily | MN1500 | Keller | Keller | sweet |
| Attiki | MN1500 | Keller | Keller | sweet |
Organic matter cycle The aim of this part of the activity was to determine the effect of Sorghum crop on soil quality. In particular, the aim was to determine if the cultivation of Sorghum helps to improve the soil organic matter content. The analytical determination of soil organic matter includes only those organic material in the fine soil sample, that is, those that accompany the soil particles through a 2- mm sieve. The most usual method estimates the organic matter content (OM) of a soil from the determination of the organic carbon, as C is the chief element present in soil organic matter. OM is usually estimated by multiplying the data of organic C concentration by a constant factor. For many years the named Van Bemmelen factor of 1.724 was used based on the assumption of a 58% C content on OM. However, this factor is highly variable from soil to soil and also between horizons in the same soil; figures from 1.724 to 2 are reported. Because of this fact, it is most appropriate to determine and report the organic C concentration in a soil as expression of OM rather than use an approximate factor.
Regarding the laboratory procedures for the determination of organic C, the dichromate oxidation methods are widely used in soil investigations, because they are simple and rapid. For this reason this method was used by some partners. On the other hand, methods for total carbon, that are characterised by their accuracy, can be also useful although it should be kept in mind that these also include the inorganic C fraction. One partner also used this approach. In order to quantify the stage of decomposition of the organic matter presented in the soil, the ratio carbon to nitrogen (C:N) is used. Fresh organic matters show high C:N figures, for instance for straw C:N of over 100 is reported while for mature manure C:N is around 25. The Kjeldahl method- based on the digestion of a soil sample in order to convert the N forms to ammonium ions, followed by a distillation and valorisation, was used together with elemental analysis of the sample.
Carbon cycle The aim of this activity was to quantify the amount of carbon that is fixed by the Sorghum crop, in order to evaluate the environmental benefits (CO2 air reduction.) of the crop. This is strictly related to organic matter studies, in relation to the experiment design; and, it is obviously related to any other sub-task in the Project, as the photoassimilation of atmospheric carbon is the basis for plant production. From the environmental point of view as well as from the agronomist concerns, it is strongly advisable to refer to the whole crop cycle instead of instantaneous measurements of the net-assimilation rate of photosynthesis, in order to quantify the environmental benefits (CO2 air reduction) of the crop. In this sense, the experimental procedure should involve a whole crop cycle and at its end, the quantification of the total C fixed by the crop. The amount of carbon fixed by the crop is determined from the carbon content of representative plant samples, referring the result to the total biomass production. The more sampled fractions, the more accurate results. For this sub-task, the plant fractions studied were: stalks, leaves, panicles and roots.
Soil erosion An experiment was carried out in Italy, where the facilities to study the soil loss caused by erosion from a slopes in the internal hills of Sicily were available. The erosion of soil is a problem which besets most of the Mediterranean European countries because of the topography of the territories, hilly and mountainous. Moreover, when the dangerous rainfall occurs at the end of summer and autumn, the soil are usually tilled, bare and ready for the sowing of winter cereals. All these facts, together with the absence of soil control measures, promote runoff and subsequently soil loss. Sweet and fiber Sorghums, being summer crops could help in controlling runoff and soil erosion during the period when the intensive rains occur, maintaining the soil covered with heavy leaf canopy during crop cultivation, helping in reducing soil water content after a rain through transpiration, and leaving crop residues after harvest. On a North-East facing slope of 26-28%, ten runoff field plots were set up, with a single plot dimension of 320 m2 (m 40 x 8). On each plot ten different cropping systems were studied, two of them including fibre Sorghum at the first and third year respectively.
Energy balance of sweet and fiber Sorghum crops This task evaluated the energy efficiency for the sweet and fibre Sorghum production as an energy budget in terms of net energy production per ha per year. The net energy budget result from the difference between the total energy outputs and the total energy inputs. The energy budget for sweet and fiber Sorghum was determined for several locations in South Europe: Lisbon (Portugal), Madrid and Badajoz (Spain), Bari, Bologna, Catania and Potenza (Italy), Athens (Greece). The value of the gross heat of combustion used in the calculations (3400 cal/g) was the same for all the locations and was obtained experimentally in Lisbon. In accounting the energy input and output the following parts of the process chain were distinguished. Energy input is the total of agronomic inputs in production phase:
The following table shows the energy standards in the cultivation phases. These energy standards represent the fossil energy used in the production of fertilisers, pesticides, seeds, machinery and in the cultivation of the energetic crop.
| N-fertiliser | 0.0386 GJ/Kg N |
| P- fertiliser | 0.0076 GJ/Kg P |
| K-fertiliser | 0.003 GJ/Kg K |
| Pesticide | 0.0515 GJ/Kg |
| Seeds | 1GJ/ha |
| Irrigation | Labour 465 Kcal/h |
| Pipes 21 000 Kcal/Kg | |
| Pump 19 941 Kcal/Kg | |
| Fuel 11 414 Kcal/l | |
| Water 0.6 Kcal/L | |
| Machinery | 8.7 GJ/ha |
Energy outputs is the total of the energy produced during combustion determined as the productivity (ton/ha) x gross heat of combustion (GJ/ton). Net Energy Gain is the difference between the total energy gain and the total agronomic inputs, producing an amount of net avoided fossil energy in GJ/ha.
Modelling nitrogen cycling in cropping systems including Sorghum The objectives of the modelling efforts within project were twofold:
The environmental impacts considered include nitrogen losses from the field, whether leaching (as nitrate) or gaseous (as ammonia and nitrous oxide). Ammonia is involved in the eutrophication of natural ecosystems and in soil acidification, whereas nitrous oxide is a greenhouse gas whose global warming power amounts to 270 times that of carbon dioxide on a molar basis. Associated deliverables include two user-friendly simulation tools, which are available to the scientific community as well as other potential users such as crop consultants or advisory bodies. The models also have the potential to provide recommendations to growers in Southern Europe to achieve high production at a minimal cost in terms of nutrient leaks in the environment.
Results
Productivity network A productivity network has been set up within the Mediterranean countries (Italy, France, Spain, Portugal and Greece) in order to quantify the potential yield of this crop in different European conditions. The field experiments carried out allowed to find out that within the Mediterranean countries, biomass yield of sweet and fibre Sorghum, could be generally higher than 20 t/ha with maximum yields around 40 t/ha in South Italy and Greece. The biomass yield of the varieties generally increased with increasing length of the growing season: the highest yields were obtained with the latest genotypes. The different duration of the growing season determined a different thermal units requirements among the studied varieties; the thermal units ranged between 1200 and 1700 degree days which indicates the possibility to define in Europe, the genotype available for each area according to the thermal regime: the early varieties more suitable for the northern areas (France, South Germany), and the late varieties, which can found high temperature conditions, could be grown in all Mediterranean countries.
Crop rotation Considering wheat production, the more convenient three years rotation was sweet Sorghum-faba bean-wheat (index 1.142), followed by soybean-fibre Sorghum- wheat (index 1.033). Considering rotations where wheat was grown on the second year, the best preceding crop was soybean (1.059),followed by faba bean (1.047). It could be noted that all crop rotations including sweet Sorghum, except sweet Sorghum- rapeseed, showed index values above 1. The worst condition for wheat production, seemed to be after fiber Sorghum (second year 0.883) and at the third year of continuous wheat (0.942). The highest production of fiber Sorghum were obtained after wheat (second year, 1.064) and after wheat-soybean (third year, 1.084). All the other combinations, apart from soybean-wheat (with Sorghum on the third year) yield index values below I.
Sweet Sorghum produced more after wheat-faba bean and wheat-soybean, compared to the continuous cultivation. Crop precession did not influence significantly sucrose content (P<0,05), therefore highest sucrose yield was reach in rotation after wheat, wheat-faba bean and wheat- soybean. Both sweet and fibre Sorghum proved to perform well in all the crop rotations involved in the experiment. Cropping cycle of both genotypes varied from 140 to 160 days, which allows for soil preparation and sowing of a fall-winter crop in succession. The good adaptability to various environmental condition and the high stability of productions, allowed for total dry matter yield exceeding 20 t/ha, of which 70% was partitioned to stems. In order to increase grain production of wheat, it seems appropriate to grow a leguminous crop (soybean, faba bean) in between of Sorghum and wheat. In fact the cultivation of wheat after Sorghum, especially after fibre Sorghum, resulted in a reduction of grain yield. Sorghum yield increased when grown after wheat cultivation, while continuous cultivation of Sorghum was negative, in particular on the third year and for the fibre genotype.
Low input Only few of the parameters measured during three years of experiments carried out in three different Italian locations, proved to be affected by input level. Total and stem dry matter productions were never affected significantly. Only total fresh biomass increased with the normal input technique, but this might could be a disadvantage, since transportation and drying costs increase with fresh matter production. Therefore it can be concluded that Sorghum (both fibre and sweet genotypes) - can be grown with a low input technique without a significant reduction in biomass production. The effect of plant population on stem and total dry biomass yield was relevant only in Piacenza. While plant density did not affect significantly LDK the significance of Density by Genotype interaction, shows that sweet Sorghum has 32% more leaves than fibre Sorghum at 10 plants per sq m, while the difference is only 20% at 20 plants per sq m.
Dry matter content of biomass at final harvest, represents an important qualitative parameter of Sorghum production. In fact, especially considering Sorghum production for energy or as raw material for industry, water content of the biomass increases costs of transportation and eventually for drying the material before industrial processing. Input level and plant density did not affect this parameter significantly. S far as total dry matter is concerned, fibre and sweet Sorghum did not differ significantly. Sweet Sorghum was in general more productive than fibre Sorghum, however the difference was larger in Piacenza and Catania. In Bologna, where sweet Sorghum was affected by lodging in 1997, fibre Sorghum was more productive on average. The largest difference between the two genotypes was recorded in Piacenza (1999) where sweet Sorghum yielded 34.2 t/ha, while fibre Sorghum only 24.9 t/ha (+37%). On average, plant population did not affect significantly total biomass production. However in Piacenza, the highest stand was always more productive (23 and 32 t/ha with 10 and 20 plants per sq m respectively).
Selection of genotypes for low N-input and low temperature Screening a collection for NUE (Nitrogen Use Efficiency) a 2-fold difference in production was found. Cultivar Fekete Magyar was about 20% more productive than any other. There was no significant difference in leaf area ratio across cultivars, but unit leaf rate was higher in the cultivars with the highest relative growth rate. Fekete Magyar again showing the higher unit leaf rate. Fekete Magyar also differed from the other cultivars in partitioning more dry matter into "stem". The higher unit leaf rate, suggests that measurement of photosynthetic rate by leaf gas exchange could provide an alternative non-destructive screen. Photosynthetic gas analysis failed to show a correlation between production at low N and light-saturated photosynthetic rate of carbon dioxide uptake.
Further analysis did however suggest another gas exchange approach to selection. The response of leaf photosynthetic rate to intercellular carbon dioxide concentration (A/ci) indicates the in vivo activity of two major sinks for leaf nitrogen: ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenol-pyruvate carboxylase (PEPCase). Analysis of the A/ci responses suggested that all cultivars had a considerable excess of PEPCase relative to amounts required to support observed maximum photosynthetic rates. Although Fekete Magyar showed the lowest apparent "over-investment" in PEPCase, the analysis suggested that it could achieve the same productivity with a third of the amount observed. In conclusion, there appears considerable variation among current cultivars that could be exploited in selecting germplasm for production with low nitrogen fertilisation. Unit leaf rate, stem partitioning and in vivo estimation of PEPCase activity appear potential screens.
Studies of the genetic basis of relevant characters in sweet Sorghum provided a mean of 20 and 32,7 polymorphisms obtained for the EcoRI/Msel and Pstl/Msel enzyme combinations, respectively. 103 bands segregated according to the Mendelian distribution and and 94 (57 PstI/MseI and 37 EcoRI/Msel) were significantly linked to the framework map, forming 10 major linkage groups spanning 1381.5.cM. The framework map was used to identify quantitative trait loci on 99 F3 progenies, each tracing back to a single F2 plant.
The following traits were investigated: sugar and sucrose percentage, dry matter and sugar yield, cycle length, plant height and germination ability at low temperatures. Simple (SIM) and composite interval mapping (CIM), were used for QTL analysis. For all the traits a total of 13 QTL were detected using CIM but only 4 using SIM indicating that CIM has a greater power in QTL detection than SIM. Significant QTLs were found for all the traits investigated, except for sugar percentage, indicating that there was no segregation of genes having major effects on this trait in the population investigated .
Selection of lines in segregating progeny from crosses: While the cross sweet x sweet produced rather deceiving results, by giving only few lines with modest advantages over the parents for both cold tolerance and yield, the other two crosses showed high potential in relation to the goals of the work. A series of lines with improved emergence ability at low temperature and high yield were identified; as to the product quality, several of them can be classified as sugar types and some as fiber ones. On the basis of these results the breeding procedure here adopted seems appropriate to the development of Sorghum for industrial use.
Seed treatment and evaluation of genotypes under low temperatures: Among the commercial genotypes studied a wide variability in terms of resistance or tolerance to low temperature have been found, both in laboratory and in field experiments. This seems to be promising for breeding programs which could allow selection of tolerant genotypes to low temperature, especially during the first stages of the season. The sensitivity of the plant to temperature thresholds, from germination to anthesis, presents different expressions, suggesting the involvement of various, at least in part independent mechanisms. For a consistent progress, more knowledge is needed on the genetics and physiology of the growth process, in particular for the critical first stages after germination. In the meanwhile appropriate selection programs for tolerance to low temperatures can be developed with some success.
The priming treatments, even if they were successful in the laboratory in promoting germination at low temperatures, did not show any beneficial effect upon plant emergence in any sowing date in field conditions. More accurate laboratory studies are needed to find out the relation between osmo-priming, seed water content, soil characteristics, air and soil temperature and seed germination
Nitrogen balance The first conclusions that can be drawn from the data are that, because of the natural microbial nitrogen mineralisation, in absence of the crop, the nitrate content of the soil increases. In all the experimental localities, sweet Sorghum species, thanks to the high nitrogen uptake, showed a positive behaviour in the control of the nitrate content in the soil. A general reduction of the nitrate content in the soil and in the soil water was observed in all the localities during the sweet Sorghum growing cycle. The capacity of sweet Sorghum to act as a catch crop is a positive consideration in terms of ability to control the nitrate leaching in the groundwater, both during the growing cycle and, because of the strong reduction during the cropping cycle, during the following winter period.
A rational irrigation programme (that avoids excess of water and dangerous nitrate leaching) stimulating the crop growth and nitrogen uptake, results in a positive reduction of the soluble form of nitrogen in the soil. This reduces the risks of excessive concentration of dangerous forms of nitrogen in the soil and in the drainage water during as well as after the growing period of Sorghum. The risk of pollution of the water table by nitrate depend mainly on the nitrate concentration of the leaching water; the amount of nitrogen lost from the soil, and the consequent reduction of fertility, depend by the volume and the nitrogen concentration of the drainage water. If this assumption is accepted and, as described above sweet and fibre Sorghum can be cultivated in Europe during the driest period of the year when the rain probability is very low and also the drainage probability is very low; therefore also the possibility of groundwater pollution and environmental impact is also very low. This occurred in all the experimental localities where an appropriate irrigation scheduling was followed.
Besides, considering the good catch capacity of this crop with respect to the nitrate in the soil and the significant reduction at the end of the cropping period, sweet Sorghum can reduce the potential risks of groundwater pollution during the following winter. The nitrate concentration at the beginning of a period, during which the nitrification by bacteria is also reduced by the thermal regime, is a guarantee of reduced environmental impact in a period of the year when the rain and water drainage is more probable. To reduce further the possibility, it can be useful to stop the irrigation some time before the end of growing cycle in order to avoid leaving the soil at the field capacity at the beginning of the rainy period, since this can buffer the first winter drainage. In any case the amount of nitrogen lost by ammonia volatilisation was less than 5% of the applied fertiliser and, therefore, it represent a negligible element of inefficiency of the nitrogen balance.
Considering the total nitrogen uptake of the plant (stalks+leaves), in all the experimental treatments a reduction of the soil nitrogen reserves were observed. In particular, the soil nitrogen deficit was quite high in the unfertilised treatment, with an average -97 kg/ha of nitrogen, while in the most fertilised treatment the average deficit was of -42 kg/ha. The results confirm the strong capacity of this species to take up nitrogen from the soil, and therefore to act as a catch crop, also when fertiliser is applied. This can be considered positive from an environmental point of view.
If a nitrogen fertiliser rate lower than 120 kg/ha is applied (this rate seems to be more appropriate for this species), particular care should be taken in the fertilisation plan of the following crop (specially in soil with a low nitrogen content) in order to avoid a dangerous reduction of the soil fertility and a unsustainable use of the soil. Considering only uptake by the stalks, a nitrogen fertilisation rate ranging from 40 to 90 kg /ha seems to be sufficient to close the apparent nitrogen balance to a point close to the neutral solution. A nitrogen fertiliser dose higher than 90 kg/ha, in the experimental context, seems not to be agronomically useful (no statistical difference in the yield of stalks was found between 60 and 120 kg/ha of nitrogen) and potentially dangerous for the environment because, if the leaves are not removed from the field, more nitrogen remains in the soil after the cultivation period than at the beginning.
Water balance With respect to well watered treatment, by reducing irrigation volume by about 40% the final yield decreased by about 30%. The effect of water stress on yield depended on the phenological stage during which it was applied. In comparison with a crop well-watered during the whole cycle (31.6 t/ha of above ground dry biomass), sweet Sorghum biomass production was reduced (36 % less) in the case of a stress occurring during the stem elongation stage. Later stages were less sensitive to soil water shortage.
The results obtained in the course of this field study provide the elements for correctly scheduling irrigation in sweet and fibre Sorghum. Firstly, irrigation becomes indispensable only when moisture in the soil drops below its wilting point. Secondly, the best stage for saving irrigation water without loosing productivity is after the fast growing period (from the second half of August), when rainfall are more frequent. Summer rainfall covered 32% of seasonal ET (under full irrigation only 4% of rainfall was used for evapotranspiration process). Irrigation should be favoured during the early stages of sweet Sorghum development. This priority should be taken into account in the case of supplemental irrigation that is extremely important in the Mediterranean regions.
Organic matter and carbon cycle From the experiments carried it can be inferred that for these experiment conditions, the biomass left by the Sorghum crop is incorporated to the soil organic matter at a low rate; the process is somewhat long-lasting, as after two years of experiments, no clear effect of the crop on the soil organic matter have been observed (neither just after the harvest nor after two years). The main conclusion that may be inferred from the carbon balance studies is that Sorghum is a promising crop from the environmental point of view. On average, one hectare of Sorghum crop can fix 30 t CO2 in its aerial biomass and 5 t CO2 in its underground biomass. Only by knowing the amount of C fixed in the aerial biomass is the amount of C fixed by the total biomass estimated as the conversion factor was steady at 1.14. This is a useful factor to estimate the benefits of the Sorghum crop for the environment, as allows to estimate the total CO2 fixed by the crop as well as the carbon amount that may be incorporated to the soil organic matter.
Energetic balance of Sweet and Fiber Sorghum crops The results show that the productivity of the fields is the most important factor in the estimation of the Net Energy gain. The productivity could be affected by the nitrogen fertilisation, the variety, the level of irrigation applied and the site and climatic conditions. The increase in nitrogen fertilisation does not affect significantly the total agronomic inputs but, in most of the sites studied, affects positively the productivities and consequently the net energy gain. The amount of water applied affects significantly the total agronomic inputs. In fact, among the energetic inputs, irrigation is the most important factor. However the Net Energy Gain is more favourable when irrigation is applied due to the fact that this item also affects positively productivity. As a result both the sweet and fibre Sorghum gave very positive energy budgets. Productivity is the most important item to be considered. So, all the conditions that could be modified in order to achieve a better yield should be applied, even if this implies an increase in the energetic inputs. However the environmental and economic aspects should also be accounted in the overall decisions. It is also important to realise that this methodology does not take into account the energy input required for transportation and conversion of biomass, so the Net Energy Gain could be overestimated.
Soil erosion The study carried out on slopes equipped with facilities to collect runoff and erosion gave interesting results in respect of suitable methods for erosion control. In the first year (1997) the late harvest - determined by the-late sowing date - after which the soil was not cultivated until the following spring, resulted in a very efficient system to control erosion, because the soil, during the intense rainfall in the period between September and December, was always covered by crop or crop residues and was not tilled. On the contrary in the third year the fiber Sorghum was harvested in October, and the soil was tilled in order to prepare the seedbed for winter cereals. This resulted in a situation in which large amounts sediment ( > 28.0 t/ha) were lost. It can be concluded that, in term of soil erosion Sorghum may be useful in maintaining the soil cover when it is sown late (July) and the soil is not tilled after harvest. The crop may be followed by a legume forage crop sown in between Sorghum before harvest (like sweet vetch) or with a sod seeding immediately after harvest.
Modelling nitrogen cycling in cropping systems including Sorghum This activity resulted in the development of two separate models, each based on a specific objective and thus of a particular form. WINSORG, the new process-based model for predicting dry matter production of Sorghum grown in southern Europe, was specifically produced for this project to allow prediction of yields over a wide area and with year-to-year variation in weather. The second model was a modified version of the CERES-Sorghum model, and was used to simulate the fate of fertiliser nitrogen (N) along with soil water and carbon dynamics, crop growth, and the emissions of gaseous N to the atmosphere. The version used was modified from its original form with introduction of new routines for the development of leaf area and the effect of water stress on leaf area increase and net photosynthesis, respectively. Further modifications were included covering the soil water and mineralisation routines to better simulate the movement of water and nitrate within the soil profile, as well as the turnover of carbon and N in the soil. in addition an ammonia volatilisation module was developed based on a simple, daily time-step approach, derived from a mechanistic soil-atmosphere model. This module simulates the chemistry of ammonium in the aqueous and gaseous phases of the soil close to the surface, and computes vertical flux within the boundary layer above the soil based on turbulent diffusivity equations.
CERES - Model performance
CERES-Sorghum was run only for a subset of the trials undertaken within the network partners in Task 3 (environmental studies); because it required a substantial amount of soil-related input data. The Barrafranca site involved two experiments on different soil types: crop rotation and low input. At all sites, monitored data comprised soil water content and, inorganic nitrogen profiles, crop biomass, green area index and nitrogen uptake, and in some instances atmospheric fluxes such as ammonia volatilisation and evapotranspiration.
Crop growth Based on the above parameters for soils and genotypes, the comparison between model outputs and measured data revealed some shortcomings with the model. First, the simulated dynamics of leaf area index (LAI) showed poor fit in the late stages with a senescence phase that set on too early according to the model. Although, the timing of the end of leaf growth was generally well predicted, the senescence factors calculated in the preceding development stage (starting from panicle initiation) appeared too drastic. Those factors account for detrimental effects of nitrogen and water deficiencies, cold temperatures (below 6 °C), and carbon budget. As could be expected, the discrepancies with LAI simulations resulted in an under-prediction of the final biomass of Sorghum, by 5 to 10 tons of dry matter/ha for the higher yields, although it was less marked in Barrafranca. However, in general the simulated time course of crop N uptake was quite realistic, which supports the idea of using CERES to simulate soil and crop N dynamics. Since CERES under-estimates crop biomass but not crop nitrogen, it is likely that it exaggerates the level of N stress for a given level of crop nutrition. It may be that the N dilution curve used here, which was originally for maize, does not apply to Sorghum.
Water and nitrogen dynamics As a general rule, the simulation of soil water and nitrogen content profiles proved less problematic than crop growth, and the simulations were close to the measured values. Both the infiltration of water down the profile and the evapotranspiration by the crop were well predicted by the model, as may be judged from the good match to observed moisture data in wet and dry regimes. Modelled nitrogen budgets the calibrated model yielded estimates of seasonal fluxes of environmental relevance such as nitrate leaching, denitrification or total water drainage below the root zone. Inevitably, as with all modelling studies, the results should be mitigated bearing in mind that some biases are associated with simulation outputs. Most significant here is the systematic under-estimation of final crop biomass, and, to a lesser extent, crop nitrogen content. However this does not imply that simulation results are irrelevant, since it is reasonable to assume the model will give a correct picture of the relative variations between treatments and locations.
WINSORG - Model performance
Above ground dry matter production simulations of WINSORG, the Windows intuitive process-based crop growth model of Sorghum grown in southern Europe, generally correlate well with observed values, particularly maximum values at the end of the growing season. The advantage of WINSORG over many empirically-based crop growth models is that it uses the C4 model equations for photosynthesis based on biochemical, mechanistic equations combined with a stomatal conductance model. WINSORG combines the high mechanistic content model of carbon gain with the well-established carbon allocation algorithms of CERES-Sorghum to produce a crop growth model based on inputs of latitude (for inputs of photosynthetically active radiation), mean daily temperature, watering regime, sowing date and soil type. Limitations of the model are that it occasionally does not match the observed data under different cultivation regimes, and that further cultivar-specific parameterisation may be required. For example, the different development rates observed between different cultivars may be parameterised by altering default values of elapsed thermal times for different growth stages.
Although WINSORG may overestimate leaf dry matter production early in the growing season, this is not as important to the Sorghum crop growth model as it would be to a growth model of other crops, partly because the main bulk of Sorghum above ground biomass is in the stems, and also because the yield is not the grain, in which case a more rigorous simulation of flag leaf development might be required.
Closer correlation between WINSORG simulations and observed data might be achieved by the insertion of a more detailed soil and root development module, which might allow simulations to differentiate between different cultivation and tillage regimes. Meanwhile, the combination of a process-based model of CO2 assimilation rates, stomatal conductance model and carbon allocation algorithms, programmed using Microsoft (TM) Visual Basic, has produced a user-friendly crop growth model that can predict the potential dry matter production of a Sorghum crop grown in various locations around southern Europe, based on latitude, mean air temperature, sowing date and soil water content.
© Copyright 2006 Policy Statements
Updated
by CPL Press
3 July, 2007
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