Biomass yield and energy balance of three perennial crops for energy use in the semi-arid Mediterranean environment

Three different lignocellulosic energy crops (a local clone of Arundo donax L., Miscanthus x giganteus Greef et Deu. and Cynara cardunculus L. var. altilis D.C. cv. “Cardo gigante inerme”) were compared over 5 years (2002–2007) for crop yield, net energy yield and energy ratio. In a hilly interior area of Sicily (Italy), two different irrigation treatments (75 and 25% of ETm restoration) and two nitrogen fertilization levels (100 and 50 kg ha⁻¹) were evaluated in a split-plot experiment. In the fourth and fifth years of the field experiment (2005–2007) no fertilizer or irrigation was used.From crop establishment to the third year, above ground dry matter yield increased over all studied factors, in A. donax from 6.1 to 38.8 t ha⁻¹ and in M. x giganteus from 2.5 to 26.9 t ha⁻¹. Fifteen months after sowing, C. cardunculus yielded 24.7 t ha⁻¹ of d.m. decreasing to 8.0 t ha⁻¹ in the third year. In the fourth and fifth years, above ground dry matter yields of all crops decreased, but A. donax and M. x giganteus still maintained high productivity levels in both years. By contrast the yield of C. cardunculus yield fell to less than 1 t ha⁻¹ of d.m. by the fourth year.Energy inputs of A. donax and M. x giganteus were higher in the year of establishment than that of C. cardunculus (34 GJ ha⁻¹ for A. donax and M. x giganteus and 12 GJ ha⁻¹ for C. cardunculus), mainly due to irrigation.Net energy yield showed low or negative values in the establishment year in A. donax and M. x giganteus. In the second and third year, net energy yield of A. donax was exceptionally high (487.2 and 611.5 GJ ha⁻¹, respectively), whilst M x giganteus had lower values (232.2 and 425.9 GJ ha⁻¹, respectively). M x giganteus attained its highest net energy yield in the fourth year (447.2 GJ ha⁻¹). Net energy yield of C. cardunculus reflected energy output of the crop, being high in the first compared to subsequent years (364.7, 277.0 and 119.2 GJ ha⁻¹, respectively for the first, second and third years).A significant effect of the different irrigation treatments was noted on all the studied parameters in all species. Conversely, only A. donax was affected by nitrogen fertilization.     

Physiological determinants of maize and sunflower grain yield as affected by nitrogen supply

Utilisation of nitrogen (N) has been closely related to increases in crop productivity. However, not all crops respond similarly and the objective of this study is to identify physiological processes that determine responses to N supply for maize and sunflower. Grain yield in maize (range: 210–1255 g m⁻²) was greater and more responsive to N supply than in sunflower (106–555 g m⁻² in carbohydrate equivalents) over a wide range of total N uptake (3–>20 g N m⁻²). In maize, differences in grain yield among levels of N supply were associated more with variation in biomass than in harvest index. In sunflower, differences in grain yield (in carbohydrate equivalents) among levels of N supply were related similarly to variation in both biomass and harvest index. The decrease in biomass production with decreasing N supply was associated with decreases in both radiation interception and radiation use efficiency (RUE). Decreased interception was due to effects of N supply on reducing canopy leaf area, whereas the reduced RUE was associated with decreased SLN. Total biomass production in maize was more responsive to N supply than in sunflower. The major determinants of the differences in response of biomass accumulation to N supply found between maize and sunflower are: (i) sunflower tends to maintain SLN with increase in partitioning of N to leaves under N limitation whereas maize tends to maintain leaf area with increase in partitioning of biomass to leaves and (ii) the ability of maize to maintain N uptake following cessation of leaf production.     

Simulation of salt and water movement and estimation of water productivity of rice crop irrigated with saline water

The HYDRUS-ID model was experimentally tested for water balance and salt build up in soil under rice crop irrigated with different salinity water (ECiw) of 0.4, 2, 4, 6, 8 and 10 dS m⁻¹ in micro-lysimeters filled with sandy loam soil. Differences of means between measured (M) and HYDRUS-1D predicted (P) values of bottom flux (Q _o) and leachate EC as tested by paired t test were not found significant at P = 0.05 and a close agreement between RMSE values showed the applicability of the HYDRUS-1D to simulate percolation and salt concentration in the micro-lysimeters under rice crop. Potential ET values of rice as obtained from CROPWAT matched well with model predicted and measured one at all ECiw treatments. The model predicted root water uptake varied from 66.1 to 652.7 mm and the maximum daily salt concentration in the root zone was 0.46, 2.3, 4.5, 6.7, 8.4 and 10.2 me cm⁻³ in 0.4, 2, 4, 6, 8 and 10 dS m⁻¹ ECiw treatments, respectively. The grain production per unit evapotranspiration ( \textWP_{\textET\texta} {\text{WP}}_{{{\text{ET}}_{\text{a}} }} ) value of 2.56 in ECiw of 0.4 dS m⁻¹ treatment declined to 1.31 with ECiw of 2 dS m⁻¹. The \textWP_{\textET\texta} {\text{WP}}_{{{\text{ET}}_{\text{a}} }} reduced to one-fifth when percolation was included in the productivity determination. Similarly, the water productivity in respect of total dry matter production (TDM) was also reduced in different treatments. Therefore, the model predicted values of water balance can be effectively utilized to calculate the water productivity of rice crop.     

Growth, yield and mineral content of Miscanthus × giganteus grown as a biofuel for 14 successive harvests

Miscanthus × giganteus, a perennial rhizomatous grass commercially used as a biofuel crop was grown in a field experiment on a silty clay loam soil for 14 years. There were 3 rates of fertilizer nitrogen (N), none (control), 60 kg N ha⁻¹ yr⁻¹ and 120 kg N ha⁻¹ yr⁻¹ as cumulative applications. The crop was harvested in winter and dry matter yield measured. N did not influence yield. Yield, which increased for the first 6 years, decreased in years 7 and 8, but then increased again and was highest in the 10th year averaging 17.7 t ha⁻¹ across all treatments. Differences in total production over the14 years were only 5% between the highest and lowest yielding treatments and averaged 178.9 t ha⁻¹ equivalent to 12.8 t ha⁻¹ yr⁻¹. In the first 10 harvests, 92% of dry matter was stem. Although the study showed N fertilizer was not required, it is considered that an application of 7 kg P ha⁻¹ yr⁻¹ and 100 kg K ha⁻¹ yr⁻¹ would avoid soil reserve depletion. Pesticides were not required every year and the crop can be considered as low input with a high level of sustainability for at least 14 years.     

Physiological Determinants of Crop Growth and Yield in Maize,Sunflower and Soybean

SUMMARY

This review focuses on the mechanisms determining yield in maize, sunflower and soybean, the three major summer crops of the Argentinean Pampas. Emphasis is given to the capture of light by the crop canopy and the processes determining grain set and grain filling. A strong correlation between grain yield and the physiological status of crops or plants at crop-specific critical periods is confirmed. This basic physiological information is used (i) to evaluate and understand the effect of agricultural management practices on crop yield and (ii) as the source of concepts and quantitative relationships for crop simulation models. Finally, we discuss the relevance of the described physiological processes for crop breeding.     

Reciprocal influence of crops and shallow ground water in sandy landscapes of the Inland Pampas

In regions with shallow water tables, ground water may have a positive (water supply) or negative (waterlogging or salinization) impact on crops. Reciprocally, crops can influence ground water, altering water table depth and chemical composition. We quantified these reciprocal influences along natural gradients of groundwater depth in flat sedimentary landscapes of the Inland Pampas occupied by wheat, soybean, and maize during two growing seasons (2006/2007 and 2007/2008). We correlated crop yield and groundwater depth maps at the field level and made direct plant, soil and groundwater observations at the stand level across topographic gradients. Water table level largely accounted for spatial crop yield variation, explaining 20–75% of their variance. An optimum groundwater depth range, where crop yields were highest, was observed for all three crop species analyzed (1.40–2.45 m for maize, 1.20–2.20 m for soybean, and 0.70–1.65 m for wheat). The areas within these optimum bands had yields that were 3.7, 3 and 1.8 times larger than those where the water table was below 4 m for wheat, maize, and soybean, respectively. As groundwater levels become shallower than these depth bands, crop yields declined sharply (∼0.05 kg m⁻² on average for every 10 cm increase in water table level), suggesting negative effects of waterlogging, root anoxia and/or salinity. Groundwater levels below these depth bands were associated with gradually declining yields, likely driven by poorer groundwater supply.     

Hydraulic Resistance of Sorghum (C4) and Sunflower (C3)

Summary

It is well known that C4 crops have a lower water requirement than C3 crops. Reasons for the difference are not well understood. Therefore, hydraulic resistance of sorghum [Sorghum bicolor (L.) Moench] (C4) and sunflower (Helianthus annuus L.) (C3) was determined to see if it might be one explanation for the lower water use of crops with the C4 photosynthetic pathway. Plants were grown under greenhouse conditions in pots with soil, which was well watered (soil matric potential of ～0 MPa) or allowed to dry (soil matric potential of ?0.038 MPa and ?0.065 MPa for sorghum and sunflower, respectively). Hydraulic resistance was calculated in two ways: (1) using the classic Ohm's law analogue, which assumes that the relation between flux (transpiration) and difference in water potentials of the soil and plant is linear and (2) using an equation that considers diurnal changes in leaf water content along with transpiration and difference in water potentials. Because change in leaf-water content during a day was small, hydraulic resistances calculated by the two methods resulted in similar values. Sorghum had a linear relationship between flux and difference in potentials (constant hydraulic resistance), but sunflower had a nonlinear one (variable hydraulic resistance). The hydraulic resistance of watered sunflower increased only slightly during a day and averaged about 40 MPa m² s mol^?1, which was 3.5 times less than that of watered or water-stressed sorghum (-140 MPa m² s mol^?1). The hydraulic resistance of water-stressed sunflower increased steeply during a day and by the end of the day it had a hydraulic resistance that approached that of sorghum.     

Simulation for response of crop yield to soil moisture and salinity with artificial neural network

In saline fields, irrigation management often requires understanding crop responses to soil moisture and salt content. Developing models for evaluating the effects of soil moisture and salinity on crop yield is important to the application of irrigation practices in saline soil. Artificial neural network (ANN) and multi-linear regression (MLR) models respectively with 10 (ANN-10, MLR-10) and 6 (ANN-6, MLR-6) input variables, including soil moisture and salinity at crop different growth stages, were developed to simulate the response of sunflower yield to soil moisture and salinity. A connection weight method is used to understand crop sensitivity to soil moisture and salt stress of different growth stages. Compared with MLRs, both ANN models have higher precision with RMSEs of 1.1 and 1.6 t ha⁻¹, REs of 12.0% and 17.3%, and R² of 0.84 and 0.80, for ANN-10 and ANN-6, respectively. The sunflower sensitivity to soil salinity varied with the different soil salinity ranges. For low and medium saline soils, sunflower yield was more sensitive at crop squaring stage, but for high saline soil at seedling stage. High soil moisture content could compensate the yield decrease resulting from salt stress regardless of salt levels at the crop sowing stage. The response of sunflower yield to soil moisture at different stages in saline soils can be understood through the simulated results of ANN-6. Overall, the ANN models are useful for investigating and understanding the relationship between crop yield and soil moisture and salinity at different crop growth stages.     

Soybean–chickpea rotation on Vertic Inceptisols: I. Effect of soil depth and landform on light interception,water balance and crop yields

Vertic Inceptisols are prone to land degradation because of excessive run-off and soil erosion during the rainy season. Productivity of soybean-based systems on these soils needs to be improved and sustained by better management of natural resources, particularly soil and water. During 1995–1997 a field study was conducted in Peninsular India on a Vertic Inceptisol watershed to study the effect of two soil depths, namely shallow (<50 cm soil depth) and medium-deep (≥50 cm soil depth) and two landform treatments, namely flat and broadbed-and-furrow (BBF) systems, on productivity and resource-use efficiency of soybean–chickpea rotation (soybean in rainy season followed by chickpea in post-rainy season). Soybean grown on flat landform on medium-deep soil had a higher leaf area index and more light interception compared to the soybean grown on the BBF landform. This resulted in an increase in mean seed yield for the flat landform (2120 kg ha⁻¹) compared to the BBF landform (1870 kg ha⁻¹). However, the landform treatments on shallow soil did not affect soybean yields. The soybean yield was higher on the medium-deep soil (1760 kg ha⁻¹) than on the shallow soil (1550 kg ha⁻¹) during 1995–1996, but were not different during 1996–1997. In both years chickpea yields and total system productivity (soybean + chickpea yields) were greater on medium-deep soil than on the shallow soil. Total run-off was higher on the flat landform (25% of seasonal rainfall) than on the BBF landform (20% of seasonal rainfall). This concomitantly increased profile water content (10–30 mm) of both soils in BBF compared to the flat landform treatment during 1995–1996, but not during 1996–1997. Deep drainage was higher in the BBF landform than in flat, especially for the shallow soil. Across landforms and soil depths, water use (evapotranspiration) by soybean–chickpea rotation during 1996–1997 ranged from 496 to 563 mm, which accounted for 54–61% of the rainfall. These results indicate that while the BBF system is useful in decreasing run-off and increasing infiltration of rainfall on Vertic Inceptisols, there is a need to increase light use by soybean on BBF during the rainy season to increase its productivity. A watershed-based farming system needs to be adopted to capture significant amount of rain water lost as run-off and deep drainage. The stored water can be used for supplemental irrigation to increase productivity of soybean-based systems leading to overall increases in resource-use efficiency, crop productivity, and sustainability.     

Productivity and resource use of direct-(drum)-seeded and transplanted rice in puddled soils in rice–rice and rice–wheat ecosystems

Conventional tilled transplanting, a widely practiced method of rice (Oryza sativa L.) establishment in puddled soils in rice–rice and rice–wheat (Triticum aestivum L.) systems in Asia, requires a large amount of labor and water, which are becoming scarce and expensive. Growing more food with the same production costs or even reduced costs and sustaining the quality of the natural resource base are a major concern. On-farm trials were conducted in Chuadanga District of Bangladesh during the wet season as monsoon rice (aman) and during the dry season as winter rice (boro) in 2006–07 to evaluate the effects of establishment methods with improved crop management on productivity, resource (land, water, and labor) use, and economic return. Rice was established by sowing in line with a drum seeder on conventional tilled puddled soils (CT-DrumR) and by transplanting in line on the day of CT-DrumR (CT-TPR1) and 30 and 35 days after CT-DrumR (CT-TPR2) in aman and boro seasons, respectively. Farmers’ usual transplanting time corresponds to the day of CT-TPR2. Grain yields in CT-DrumR and CT-TPR2 were similar but the crop occupied the main field 22–24 days longer in CT-DrumR than in CT-TPR2, resulting in lower productivity (45 kg grain ha⁻¹ day⁻¹ vs. 55 kg grain ha⁻¹ day⁻¹) in both seasons. Drum-seeded rice matured earlier by 8 and 11 days, received 12% and 6% less irrigation water, saved 19 and 24 person-days ha⁻¹, and gave higher gross margins of 6% and 4% but input costs increased by 20% and 12% than CT-TPR2 in aman and boro seasons, respectively. There is a need to examine these benefits of drum-seeded rice in relation to the feasibility of adoption by farmers.     

Simulation analysis of lucerne–wheat crop rotation on the Loess Plateau of Northern China

The Agricultural Production System Simulator (APSIM) was parameterised and tested against datasets from two field experiments being conducted on Heilu soil at the Qingyang Research Station, Gansu, China as to investigate long-term lucerne productivity and management options of reducing impact of lucerne on winter wheat yield in a lucerne–wheat rotation system. With minimal parameterisation and configuration of the APSIM-Lucerne module, APSIM was able to simulate phenological development and seasonal growth of winter-dormant lucerne cultivar, Longdong compared with the observed data. Flowering date was accurately simulated using the established relationship between accumulated thermal time and mean photoperiod. After the APSIM-Lucerne module was configured for the seasonal variation in RUE (radiation use efficiency), the model simulated lucerne seasonal biomass production over three growing seasons in the continuous lucerne treatment with a root mean squared deviation (RMSD) of 1132 kg/ha (30% of the mean observed biomass). In the treatment where lucerne was removed in August 2001 and two winter wheat crops were sown and harvested in 2001/2002 and 2002/2003 growing seasons, APSIM simulated winter wheat crop biomass in both growing seasons with a RMSD of 1420 kg/ha (20% of the mean observed crop biomass). Wheat grain yield was simulated with a RMSD of 918 kg/ha (27% of the mean observed grain yield). Using measurements of drained upper limit (DUL) and lower limit (LL), and standard soil evaporation and runoff parameters, the model was able to simulate soil water dynamics and water use by lucerne in the lucerne-fallow, continuous lucerne and lucerne–wheat treatments.     

Causes of variation in the rate of increase of wheat harvest index

Several crop simulation models calculate grain yield by assuming that the rate of change of harvest index (δHI/δt) is constant (at rate k) during grain growth. Such behaviour has been identified in many crops, although the literature contains many examples of variations in k. The concept is useful if it approximates the truth in most circumstances, or if departures from both linearity and constancy are predictable from either the environment or the state of the crop. In this paper we examine the hypothesis that much of the variation in k is related to both crop biomass at the start of grain filling (BGF) and the crop growth rate during grain growth (CGF). Calculations using simple partitioning rules indicated that both factors are important. We showed that k increases rapidly as BGF decreases below about 9.0 Mg DM/ha, but decreases only slowly with increases of BGF above 9.0 Mg DM/ha. The analysis also showed that the increase in HI with time is quadratic rather than linear. We analysed data from 68 field grown wheat crops with variation in cultivar, location, irrigation, ambient CO₂ concentration and sowing dates. These showed an almost three-fold variation in k (0.0058–0.0164 day⁻¹). Across all data sets, there was a negative linear relationship (y = 0.02 − 0.0006x, R² = 0.41, p < 0.001) between k and BGF. Overall, these results suggest that some of the variation in k can be accounted for by differences in BGF. We suggest that any model that uses harvest index change as a basis for calculating yield should account, at least, for variations in BGF.     

The herbage productivity of tall fescue in the Pampas region of Argentina is correlated to its ecological niche

Since its introduction in the 1950s, tall fescue has spread throughout the Pampas region of Argentina to become naturalized in much of this region. Its annual forage mass productivity shows significant differences among sites. In this work, we hypothesized that, under dryland conditions, the annual forage mass productivity is related to the probability that the corresponding site is within the ecological niche of the species. To test this hypothesis, the spontaneous occurrence of tall fescue was surveyed in an area of 440 000 km². The abundance of tall fescue was verified on roadsides of national and provincial roads, in systematic sampling points spaced every 50 km. Subsequently, the ecological niche of the species was estimated with the Maxent software, based on the environmental information available. Additionally, the bioclimatic variables with greatest influence in determining the ecological niche were identified. Based on the information of comparative yield trials of tall fescue cultivars, a correlation analysis between annual forage mass productivity and the likelihood of ecological niche was performed. We found that the realized ecological niche of tall fescue in the Pampas region covers an area of 237 000 km² where the average minimum temperature of the coldest month is between 2·6 and 3·2°C and the rainfall in the driest quarter is 100 mm or higher. We also found that there were significant correlations between annual forage mass productivity and the probability of realized ecological niche. Therefore, in areas with low probability of realized ecological niche, other forage species could be a better option than tall fescue to obtain a high and stable annual forage mass productivity.     

The suitability of non-legume cover crops for inorganic soil nitrogen immobilisation in the transition period to an organic no-till system

The aim of the study was to evaluate non-legume cover crops for growing no-till grain legumes in organic farming systems. Evaluated cover crops should be able to suppress weed growth, reduce plant available nitrogen in the soil and produce large amounts of biomass with slow N mineralisation. Six non-legume species; spring rye (Secale cereale L.), black oat (Avena sativa L.), sunflower (Helianthus annuus L.), white mustard (Sinapis alba L.), buckwheat (Fagopyrum esculentum Moench) and hemp (Cannabis sativa L.) were tested. Plots with organic fertiliser (50 kg N ha^?1) and without fertiliser incorporation at three locations in south-east Germany were trialled and the cover crops’ ability to produce biomass and accumulate N in plant compartments was evaluated. The N mineralisation from stem and leaf material was simulated using the STICS model. The biomass production ranged from 0.95 to 7.73 Mg ha^?1, with fertiliser increasing the total biomass at locations with low-N status. Sunflower consistently displayed large biomass and N accumulation at all locations and fertiliser variations, although not always significantly more than other species. Most N was stored in sunflower leaf material, which can be easily mineralised making it less suited as cover crop before no-till sown spring grain legumes. Rye, which produced slightly less biomass, but accumulated more N in the stem biomass, would be better suited than sunflower in this type of system. The N mineralisation simulation from rye biomass indicated long N immobilisation periods potentially improving weed suppression within no-till sown legume cash crops.     

Yield potential and water use efficiency of aerobic rice (Oryza sativa L.) in Japan

Intensive rice farming in aerobic soil, referred to herein as aerobic rice, can greatly reduce the water input compared to that of flooded rice cultivation. The objective of this study was to compare the potential productivity of aerobic rice and flooded rice using high-yielding varieties at two locations in Japan in two successive years. In aerobic fields, the total amount of water supplied (irrigation plus rainfall) was 800–1300 mm. The soil water potential at 20-cm depth averaged between −15 and −30 kPa each growing season, but frequently reached −60 kPa. The average yield under aerobic conditions was similar to or even higher than that achieved with flooded conditions (7.9 t ha⁻¹ in 2007 and 9.4 t ha⁻¹ in 2008 for aerobic versus 8.2 t ha⁻¹ for flooded). The average water productivity under aerobic conditions was 0.8–1.0 kg grain m⁻³ water, slightly higher than common values in the literature. The super-high-yielding cultivar Takanari achieved yields greater than 10 t ha⁻¹ with no yield penalty under aerobic conditions in 3 out of 4 experiments. The favorable agronomic characteristic of Takanari was its ample sink capacity (grain number × grain weight). In conclusion, high-productivity rice cultivation in aerobic soil is a promising technology for water conservation. With continued breeding, future aerobic rice varieties will possess large numbers of spikelets and sufficient adaptation to aerobic conditions such that they will consistently achieve yields comparable to the potential yield of flooded rice.     

Phosphorus accumulation and depletion in soils in wheat–maize cropping systems: Modeling and validation

Long-term (over 15 years) winter wheat (Triticum aestivum L.)–maize (Zea mays L.) crop rotation experiments were conducted to investigate the accumulation of phosphorus (P) at five sites differing geographically and climatically in China. The results showed that, in soils without P added, the concentration of soil P extracted by 0.5 mol L⁻¹ NaHCO₃ at pH 8.5 (Olsen-P) decreased with cultivation time until about 3 mg kg⁻¹, afterwards it remained constant. The trend of decrease in Olsen-P in soils without P added could be described by an exponential function of time. The concentration of Olsen-P in soils with P fertilizers increased with cultivation time and the model of accumulation of Olsen-P in soils could be described using P application rate, crop yield and soil pH. The accumulation rate of Olsen-P in the long-term wheat–maize crop rotation experiments was 1.21 mg kg⁻¹ year⁻¹ on average. If the target yield of wheat and maize is 10 ton ha⁻¹ in the soil with pH 8, the increasing rates of Olsen-P in soils as estimated by the model will be 0.06, 0.36, 0.66, 0.95, 1.25 and 1.55 mg kg⁻¹ year⁻¹ when P application rates are 30, 40, 50, 60, 70 and 80 kg P ha⁻¹ year⁻¹, respectively. The models of accumulation of Olsen-P in soils were validated independently and could be used for the accurate prediction of accumulation rate of Olsen-P in soils with wheat–maize rotation systems. Also the application of the model was discussed for best management of soil P in agricultural production and environment protection.     

Modelling the nitrogen-driven trade-off between nitrogen utilisation efficiency and water use efficiency of wheat in eastern Australia

The nitrogen-driven trade-off between nitrogen utilisation efficiency (yield per unit nitrogen uptake) and water use efficiency (yield per unit evapotranspiration) is widespread and results from well established, multiple effects of nitrogen availability on the water, carbon and nitrogen economy of crops. Here we used a crop model (APSIM) to simulate the yield, evapotranspiration, soil evaporation and nitrogen uptake of wheat, and analysed yield responses to water, nitrogen and climate using a framework analogous to the rate-duration model of determinate growth. The relationship between modelled grain yield (Y) and evapotranspiration (ET) was fitted to a linear-plateau function to derive three parameters: maximum yield (Y_max), the ET break-point when yield reaches its maximum (ET^#), and the rate of yield response in the linear phase (ΔY/ΔET). Against this framework, we tested the hypothesis that nitrogen deficit reduces maximum yield by reducing both the rate (ΔY/ΔET) and the range of yield response to evapotranspiration, i.e. ET^# − E_s, where E_s is modelled median soil evaporation.     

Simulation of soil drying induced phosphorus deficiency and phosphorus mobilization as determinants of maize growth near tree lines on a Ferralsol

Understanding the effect trees have on the growth of crops requires an understanding of the multiple interacting processes that determine resource uptake by the crops. On a Ferralsol in sub-humid western Kenya maize (Zea mays L.) growth was primarily limited by phosphorus availability. We observed that maize growth near grevillea (Grevillea robusta A. Cunn.) tree lines was strongly reduced, while maize growth was slightly increased near cassia (Cassia spectabilis DC (syn. Senna spectabilis, DC, H.S. Irwin and R.C. Barneby). This was contrary to expectations because grevillea has a relatively low nutrient demand while Cassia has a relatively high nutrient demand.We compared maize growth in an experiment with simulations using the mechanistic tree–crop interaction model WaNuLCAS. The model simulations showed that the measured 30–40% decrease in maize growth near the Grevillea tree line was due to 0.025 m³ m⁻³ lower soil water contents (at mean levels of 0.35 m³ m⁻³ and high pF). This was not due to direct water limitation. The lower soil water content caused decreased P diffusion to roots and a cumulative decrease in crop root-growth and a concomitant decrease in crop growth over time.Measured maize yield near Cassia was 115%, unaffected by trees. Model simulations predicted it should be reduced to 80% due to direct competition for P between tree and crop. This suggests that rhizosphere modifications measured near Cassia roots probably supplied P to the tree itself and also to the maize crop.On P-limiting tropical soils, it is important to prevent soil drying to avoid soil drying induced P deficiency. In these conditions tree species that are able to mobilize P can prevent competition with the crop and may even increase crop performance.     

Relations between summer crops and ground cover legumes in a subtropical environment 3. Soil water use under sorghum and sunflower

Vigna trilobata (L.) Verdc. and Medicago scutellata L. are short herbaceous legumes that help to reduce soil erosion when grown in association with row crops in the sub-humid/semi-arid subtropics. Soil water use and root distribution beneath sorghum (Sorghum bicolor (L.) Moench) and sunflower (Helianthus annuus L.) with and without V. trilobata were measured on a vertisol in South East Queensland. Sorghum was sown in October and January at densities of 10 and 5 plants m⁻² and sunflower at densities of 5 and 2.5 plants m⁻². V. trilobata seed was broadcast at 150 seeds m⁻². Pure stands of V. trilobata and M. scutellata were also sown.Soil profiles were uniformly wet to at least 2 m depth at each sowing date and remained so for the first 40 days of each growth cycle. Thereafter profiles dried rapidly over the next 40 days, providing opportunities for assessing soil water use. Subsequent rain restored and sustained profile water content until crop maturity.Undersown V. trilobata hastened soil drying, resulting in the extraction of 21 mm more water by maturity of the October sown crops. In pure stands, V. trilobata extracted water at the same rate as sunflower from the top 1.0 m of the soil profile, but not from the deeper layers. However, pure stands of M. scutellata extracted much less soil water than V. trilobata, with negligible extraction from below 0.8 m. Sunflower extracted 48 mm more water than sorghum in the January sowing because it had more roots and extracted more water from below 1.4 m.The capacity of V. trilobata to grow deep roots allows it to compete strongly for soil water and decrease yield of the main crop. Thus, a shallow-rooting plant would be preferred as a ground cover.     

Two perennial legumes (Astragalus adsurgens Pall. and Lespedeza davurica S.) adapted to semiarid environments are not as productive as lucerne (Medicago sativa L.), but use less water

Perennial forage legumes, particularly lucerne (Medicago sativa L.), play a significant role in crop/livestock mixed farming systems in the semiarid region of the Loess Plateau of China as stock feed and a source of nitrogen for subsequent crops. However, there is evidence that lucerne reduces soil water deep in the soil profile, thereby reducing subsequent crop productivity. From 2004 to 2010, this study evaluated the forage productivity and water use of two locally adapted perennial legume species, milk vetch (Astragalus adsurgens Pall.) and bush clover (Lespedeza davurica S.), compared with lucerne. The 7‐year total and average annual forage yield of milk vetch were 56 and 8 t ha^?1 and bush clover was 42 and 6 t ha^?1, respectively, significantly lower than lucerne at 91 and 13 t ha^?1. However, despite lower water‐use efficiencies (16 and 12 kg ha^?1 mm^?1 for milk vetch and bush clover, respectively, compared to 22 kg ha^?1 mm^?1 for lucerne), the total 7‐year water use in milk vetch and bush clover was 3500 mm and 3490 mm, respectively, which was 135–140 mm less than lucerne. After 7 years, lucerne had extracted water from the upper 5 m soil, whereas bush clover used water mainly from the upper 2 m of the soil profile and milk vetch still had some water available below 3 m. We conclude that while the locally adapted forage legumes were not as productive as lucerne as a source of fodder in mixed cropping/livestock system in this region, they use less water, which may be advantageous in drier regions.