Effects of crop growth and soil treatments on microbial C,N, and P in dry tropical arable land

We studied the dynamics of microbial C, N, and P in soil cropped with rice (Oryza sativa) and lentils (Lens culinaris) in a dryland farming system. The crop biomass and grain yield were also studied. The microbial biomass and its N and P contents were larger under the lentil than under the rice crop. Microbial nutrients decreased as the crops grew and then increased again. Farmyard manure and NPK fertilizer applications increased the level of microbial nutrients, crop biomass, and grain yield by 35–80%, 55–85%, and 74–86%, respectively. However, these applications had no significant effect on most of the soil physicochemical properties in the short term. The microbial biomass was correlated with the crop biomass and grain yield. The calculated flux of N and P through the microbial biomass ranged from 30–45 and 10–19 kg ha^-1 year^-1, respectively. Cultivation of a cereal crop followed by a leguminous crop sustains higher levels of microbial nutrients and hence greater fertility in impoverished tropical arable soils. The soil microbial biomass appears to contribute significantly to crop productivity by releasing nutrients, and applications of manure, either alone or with fertilizers, promote this effect more strongly than the application of NPK fertilizers alone.     

15N studies on the transfer of legume-fixed nitrogen to associated cereals in intercropping systems

Summary An attempt has been made to estimate quantitatively the amount of N fixed by legume and transferred to the cereal in association in intercropping systems of wheat (Triticum aestivum L.) — gram (Cicer arietinum L.) and maize (Zea mays L.) —cowpea (Vigna unguiculate L.) by labelling soil and fertilizer nitrogen with ¹⁵N. The intercropped legumes have been found to fix significantly higher amounts of N as compared with legumes in sole cropping if the intercropped cereal-legume received the same dose of fertilizer N as the sole cereal crop. But when half of the dose of the fertilizer N applied to sole cereal crop was received by intercropped plants, the amount of N fixed by legumes in association with cereals was significantly less than that fixed by sole legumes. Under field conditions 28% of the total N uptake by maize (21.2 kg N ha^–1) was of atmospheric origin and was obtained by transfer of fixed N by cowpea grown in association with maize. Under greenhouse conditions gram and summer and monsoon season cowpea have been found to contribute 14%–20%, 16% and 32% of the total N uptake by associated wheat and summer and monsoon maize crops, respectively. Inoculation of cowpea seeds with Rhizobium increased both the amount of N fixed by cowpea and transferred to maize in intercropping system.     

Effects of potato–grain rotations on soil erosion, carbon dynamics and properties of rangeland sandy soils

The potential for wind erosion in South Central Colorado is greatest in the spring, especially after harvesting of crops such as potato (Solanum tuberosum L.) that leave small amounts of crop residue in the surface after harvest. Therefore it is important to implement best management practices that reduce potential wind erosion and that we understand how cropping systems are impacting soil erosion, carbon dynamics, and properties of rangeland sandy soils. We evaluate the effects of cropping systems on soil physical and chemical properties of rangeland sandy soils. The cropping system included a small grain–potato rotation. An uncultivated rangeland site and three fields that two decades ago were converted from rangeland into cultivated center-pivot-irrigation-sprinkler fields were also sampled. Plant and soil samples were collected in the rangeland area and the three adjacent cultivated sites. The soils at these sites were classified as a Gunbarrel loamy sand (Mixed, frigid Typic Psammaquent). We found that for the rangeland site, soil where brush species were growing exhibited C sequestration and increases in soil organic matter (SOM) while the bare soil areas of the rangeland are losing significant amounts of fine particles, nutrients and soil organic carbon (SOM-C) mainly due to wind erosion. When we compared the cultivated sites to the uncultivated rangeland, we found that the SOM-C and soil organic matter nitrogen (SOM-N) increased with increases in crop residue returned into the soils. Our results showed that even with potato crops, which are high intensity cultivated cropping systems, we can maintain the SOM-C with a rotation of two small grain crops (all residue incorporated) and one potato crop, or potentially increase the average SOM-C with a rotation of four small grain crops (all residue incorporated) and one potato crop. Erosion losses of fine silt and clay particles were reduced with the inclusion of small grains. Small grains have the potential to contribute to the conservation of SOM and/or sequester SOM-C and SOM-N for these rangeland systems that have very low C content and that are also losing C from their bare soils areas (40%). Cultivation of these rangelands using rotations with at least two small grain crops can reduce erosion and maintain SOM-C and increasing the number of small grain crops grown successfully in rotation above two will potentially contribute to C and N sequestration as SOM and to the sequestration of macro- and micro-nutrients.     

Cassava-legume intercrop: II. Effects of relative planting dates of legumes on the productivity of legumes

ABSTRACT

The degree of complementarity vis-à-vis competition amongst the component crops, which is influenced by their relative planting dates, may affect the productivity of intercrop systems. This study assessed the effect of the relative planting dates of legumes on yield and yield components of three legume species. Field experiments were conducted in two consecutive years at two sites with contrasting soil types. Grain yield and yield components were determined at harvest maturity. A 4-weeks delay in incorporating legumes caused total yield loss of cowpea at both sites, decreased chickpea yield by 82% at the clay soil site, and led to negligible yield of Bambara groundnut in loamy sand soil. In contrast, sowing legumes 2 weeks after cassava decreased grain yield of cowpea (both sites), chickpea (clay soil) and Bambara groundnut (loamy sand soil). Intercropping decreased grain yield of chickpea (year 1) and Bambara (year 2) at the clay and loamy sand soil sites, respectively, but had no effect on cowpea yield. Although planting the legumes same time with cassava gave the highest grain yield, we suggest more studies, including sowing the legumes prior to planting cassava, before making categorical recommendations.     

Nitrate leaching from organic arable crop rotations: effects of location, manure and catch crop

Abstract. Nitrate leaching from crop rotations supporting organic grain production was investigated from 1997 to 2000 in a field experiment at three locations in Denmark on different soil types. Three experimental factors were included in the experiment in a factorial design: (1) proportion of N₂-fixing crops in the rotation (crop rotation), (2) catch crop (with and without), and (3) manure (with and without). Three, four-course rotations were compared, two at each location. The nitrate leaching was measured using ceramic suction cells. Leaching losses from the crop rotation with grass–clover green manure and without catch crops were 104, 54 and 35 kg N ha⁻¹ yr⁻¹ on the coarse sand, the loamy sand, and the sandy loam, respectively. There was no effect of manure application or time of ploughing-in the grass–clover green manure crop on the accumulated nitrate leaching from the entire rotation. Catch crops reduced nitrate leaching significantly, by 30–38%, on the sandy soils. At all locations catch crops reduced the annual averaged nitrate concentration to meet drinking water quality standards in the crop rotation with green manure. On the coarse sand there was a time lag between the onset of drainage and the start of N-uptake by the catch crop.     

Green manure production of Azolla microphylla and Sesbania rostrata and their long-term effects on rice yields and soil fertility

Summary Azolla spp. and Sesbania spp. can be used as green manure crops for wetland rice. A long-term experiment was started in 1985 to determine the effects of organic and urea fertilizers on wetland rice yields and soil fertility. Results of 10 rice croppings are reported. Azolla sp. was grown for 1 month and then incorporated before transplanting the rice and 3–4 weeks after transplanting the rice. Sesbania rostrata was grown for 7–9 weeks and incorporated only before transplanting the rice. Sesbania sp. grew more poorly before dry season rice than before wet season rice. Aeschynomene afraspera, which was used in one dry season rice trial, produced a larger biomass than the Sesbania sp. The quantity of N produced by the Azolla sp. ranged from 70 to 110 kg N ha^-1. The Sesbania sp. produced 55–90 kg N ha^-1 in 46–62 days. Rice grain yield increases in response to the green manure were 1.8–3.9 t ha^-1, similar to or higher than that obtained in response to the application of 60 kg N ha^-1 as urea. Grain production per unit weight of absorbed N was lower in the green manure treatments than in the urea treatment. Without N fertilizer, N uptake by rice decreased as the number of rice crops increased. For similar N recoveries, Sesbania sp. required a lower N concentration than the Azolla sp. did. Continuous application of the green manure increased the organic N content in soil on a dry weight basis, but not on a area basis, because the application of green manure decreased soil bulk density. Residual effects in the grain yield and N uptake of rice after nine rice crops were found with a continuous application of green manure but not urea.     

Influence of crop residue management, cropping system and N fertilizer on soil N and C dynamics and sustainable wheat (Triticum aestivum L.) production

Management of N is the key for sustainable and profitable wheat production in a low N soil. We report results of irrigated crop rotation experiment, conducted in the North West Frontier Province (NWFP), Pakistan, during 1999–2002 to evaluate effects of residue retention, fertilizer N application and mung bean (Vigna radiata) on crop and N yields of wheat and soil organic fertility in a mung bean–wheat sequence. Treatments were (a) crop residue retained (+residue) or (b) removed (−residue), (c) 120 kg N ha⁻¹ applied to wheat, (d) 160 kg N ha⁻¹ to maize or (e) no nitrogen applied. The cropping system was rotation of wheat with maize or wheat with mung bean. The experiment was laid out in a spit plot design. Postharvest incorporation of crop residues significantly (p < 0.05) increased the grain and straw yields of wheat during both years. On average, crop residues incorporation increased the wheat grain yield by 1.31 times and straw yield by 1.39 times. The wheat crop also responded strongly to the previous legume (mung bean) in terms of enhanced grain yield by 2.09 times and straw yield by 2.16 times over the previous cereal (maize) treatment. Application of fertilizer N to previous maize exerted strong carry over effect on grain (1.32 times) and straw yield (1.38 times) of the following wheat. Application of N fertilizer to current wheat produced on average 1.59 times more grain and 1.77 times more straw yield over the 0 N kg ha⁻¹ treatment. The N uptake in wheat grain and straw was increased 1.31 and 1.64 times by residues treatment, 2.08 and 2.49 times by mung bean and 1.71 and 1.86 times by fertilizer N applied to wheat, respectively. The soil mineral N was increased 1.23 times by residues, 1.34 times by mung bean and 2.49 times by the application of fertilizer N to wheat. Similarly, the soil organic C was increased 1.04-fold by residues, 1.08 times by mung bean and 1.00 times by the application of fertilizer N. We concluded that retention of residues, application of fertilizer N and involvement of legumes in crop rotation greatly improves the N economy of the cropping system and enhances crop productivity in low N soils.     

Comparative effects of applying leguminous and non-leguminous green manures and inorganic N on biomass yield and nitrogen uptake in flooded rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The beneficial role of green manures in rice production is generally ascribed to their potential of supplying plant nutrients, particularly nitrogen (N). However, the mechanisms through which green manures enhance the crop productivity are poorly understood. Pot experiments were conducted using a ¹⁵N-tracer technique: (1) to compare the biomass production potential of sesbania (Sesbania aculeata Pers.) and maize (Zea mays L.) as green manuring crops for lowland rice and (2) to compare the effect of the two types of green manure and inorganic N on the dry matter accumulation and N uptake by two rice (Oryza sativa L.) cultivars, viz. IR-6 and Bas-370. Although maize produced three times higher shoot biomass compared with sesbania, the latter showed higher N concentration; and thus the total N yield was similar in the two types of plants. Applying the shoot material of the two plants to flooded rice significantly enhanced the dry matter yield and N uptake by the two rice cultivars, the positive effects generally being more pronounced with sesbania than with maize amendment. The difference in the growth-promoting potential of the two plant residues was related more to an increased uptake of the native soil N rather than to their direct role as a source of plant-available N. A positive added nitrogen interaction (ANI) was observed due to both plant residues, the effect was much more pronounced with the application of sesbania than with maize residues. In both rice cultivars, inorganic N also caused a substantial ANI, particularly at higher application rate. Losses from the applied N were 2–3 times lower from sesbania, compared with maize treatment. Green manuring with sesbania also caused much lower N losses than the inorganic N applied at equivalent or higher rates. The overall benefit of green manuring to rice plants was higher than inorganic N applied at comparable rates. The two rice cultivars differed in their response to green manuring, IR-6 generally being more responsive than Bas-370.     

Nitrogen contribution to wetland rice by green manuring with Sesbania spp. in an alkaline soil

Summary Two annual species of Sesbania, S. aculeata and Sesbania sp. PL Se-17, were field evaluated as green manure for wetland rice in an alkaline soil. The two species were raised as a catch crop during summer in a wheat-rice rotation, and added as 24.7 and 20.8 t ha^–1 of green matter, 116 and 98 kg N ha^–1, respectively, after 45 days of growth. For the optimum green manuring effect on rice grain yield and N uptake, S. aculeata required 5 days of decomposition (after turning in and before rice transplantation), whereas no decomposition period was necessary for Sesbania sp. PL Se-17. The effect on grain yield and N uptake of rice was equivalent to an application of 122 and 78 kg ha^–1 of chemical N, for the two species, respectively. There was no residual effect of the green manuring on the soil N status after rice harvest.     

Effect of Incorporation of Crop Residues on a Maize–Groundnut Sequence in the Humid Tropics. II. Soil Physical and Chemical Properties

Abstract

A two‐year study (1997–1999) was conducted on a sandy clay loam (Typic Paleudult) at the experimental farm of the Universiti Putra to determine the effects of application of crop residues on changes of some soil properties in a maize (Zea mays L.)–groundnut (Arachis hypogaea) crop rotation system. Five crops of a rotation of sweet corn–groundnut–sweet corn–groundnut–sweet corn were sown with three treatments: recommended inorganic fertilizer [nitrogen (N), phosphorus (P), and potassium (K)] with crop residue (T1), recommended inorganic fertilizer without crop residues (T2) or one‐half of the recommended inorganic fertilizer with crop residues combined with 10 t ha^?1 of chicken manure (T3). Soil organic carbon (OC), soil water content and soil bulk density were not significantly changed. Application of crop residues for two years increased cation exchange capacity (CEC) whereas supplementing crop residues with CM had significantly increased soil pH of the topsoil. Phosphorous in manure treatment had moved down the soil profile, which might cause eutriphication of under ground water, particularly during the rainy season. Based on this work, incorporation of crop residues could be a beneficial practice for improving the fertility of acid soils.     

Reclamation and salt leaching efficiency for tile drained saline‐sodic soil using marginal quality water for irrigating rice and wheat crops

Due to increased population and urbanization, freshwater demand for domestic purposes has increased resulting in a smaller proportion for irrigation of crops. We carried out a 3‐year field experiment in the Indus Plains of Pakistan on salt‐affected soil (EC_e 15·67–23·96 dS m⁻¹, pH_s 8·35–8·93, SAR 70–120, infiltration rate 0·72–0·78 cm h⁻¹, ρ _b 1·70–1·80 Mg m⁻³) having tile drainage in place. The 3‐year cropping sequence consisted of rice (Oryza sativa L.) and wheat (Triticum aestivum L.) crops in rotation. These crops were irrigated with groundwater having electrical conductivity (EC) 2·7 dS m⁻¹, sodium adsorption ratio (SAR) 8·0 (mmol L⁻¹)^1/2 and residual sodium carbonate (RSC) 1·3 mmol_c L⁻¹. Treatments were: (1) irrigation with brackish water without amendment (control); (2) Sesbania (Sesbania aculeata) green manure each year before rice (SM); (3) applied gypsum at 100 per cent soil gypsum requirement (SGR) and (4) applied gypsum as in treatment 3 plus sesbania green manure each year (GSM). A decrease in soil salinity and sodicity and favourable infiltration rate and bulk density over pre‐experiment levels are recorded. GSM resulted in the largest decrease in soil salinity and sodicity. There was a positive relationship between crop yield and economic benefits and improvement in soil physical and chemical properties. On the basis of six crops, the greatest net benefit was obtained from GSM. Based on this long‐term study, combined use of gypsum at 100 per cent soil gypsum requirement along with sesbania each year is recommended for soil amelioration and crop production. Copyright © 2010 John Wiley & Sons, Ltd.     

Modeling of Interactive Effects of Rainfall,Evaporation, Soil Temperature,and Soil Fertility for Sustainable Productivity of Sorghum + Cowpea and Cotton + Black Gram Intercrops under Rotation Trials in a Rain-Fed Semi-arid Vertisol

Long-term effects of the different combinations of nutrient-management treatments were studied on crop yields of sorghum + cowpea in rotation with cotton + black gram. The effects of rainfall, soil temperature, and evaporation on the status of soil fertility and productivity of crops were also modeled and evaluated using a multivariate regression technique. The study was conducted on a permanent experimental site of rain-fed semi-arid Vertisol at the All-India Coordinated Research Project on Dryland Agriculture, Kovilpatti Centre, India, during 1995 to 2007 using 13 combinations of nutrient-management treatments. Application of 20 kg nitrogen (N) (urea) + 20 kg N [farmyard manure (FYM)] + 20 kg phosphorus (P) ha^?1 gave the greatest mean grain yield (2146 kg ha^?1) of sorghum and the fourth greatest mean yield (76 kg ha^?1) of cowpea under sorghum + cowpea system. The same treatment maintained the greatest mean yield of cotton (546 kg ha^?1) and black gram (236 kg ha^?1) under a cotton + cowpea system. When soil fertility was monitored, this treatment maintained the greatest mean soil organic carbon (4.4 g kg^?1), available soil P (10.9 kg ha^?1), and available soil potassium (K) (411 kg ha^?1), and the second greatest level of mean available soil N (135 kg ha^?1) after the 13-year study. The treatments differed significantly from each other in influencing soil organic carbon (C); available soil N, P, and K; and yield of crops attained under sorghum + cowpea and cotton + black gram rotations. Soil temperature at different soil depths at 07:20 h and rainfall had a significant influence on the status of soil organic C. Based on the prediction models developed between long-term yield and soil fertility variables, 20 kg N (urea) + 20 kg N (FYM) + 20 kg P ha^?1 could be prescribed for sorghum + cowpea, and 20 kg N (urea) + 20 kg N (FYM) could be prescribed for cotton + black gram. These combinations of treatments would provide a sustainable yield in the range of 1681 to 2146 kg ha^?1 of sorghum, 74 to 76 kg ha^?1 of cowpea, 486 to 546 kg ha^?1 of cotton, and 180 to 236 kg ha^?1 of black gram over the years. Beside assuring greater yields, these soil and nutrient management options would also help in maintaining maximum soil organic C of 3.8 to 4.4 g kg^?1 soil, available N of 126 to 135 kg ha^?1, available soil P of 8.9 to 10.9 kg ha^?1, and available soil K of 392 to 411 kg ha^?1 over the years. These prediction models for crop yields and fertility status can help us to understand the quantitative relationships between crop yields and nutrients status in soil. Because black gram is unsustainable, as an alternative, sorghum + cowpea could be rotated with cotton for attaining maximum productivity, assuring sustainability, and maintaining soil fertility on rain-fed semi-arid Vertisol soils.     

Nitrogen mineralization and availability of mixed leguminous and non-leguminous cover crop residues in soil

Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitrogen (N) leaching during wet seasons, leguminous cover crops such as hairy vetch (Vicia villosa) improve soil N fertility for succeeding crops. With mixtures of grasses and legumes as cover crop, the goal of reducing N leaching while increasing soil N availability for crop production could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N uptake by cereal rye. Both cereal rye and annual ryegrass contained low total N, but high water-soluble carbon and carbohydrate, compared with hairy vetch. Decreasing the proportion of hairy vetch in the mixed residues decreased net N mineralization, rye plant growth and N uptake, but increased the crossover time (the time when the amount of net N mineralized in the residue-amended soil equalled that of the non-amended control) required for net N mineralization to occur. When the hairy vetch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significantly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N and the potential initial N mineralization rate for rye biomass (r=0.93, P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportion of rye or annual ryegrass when mixed with residues of hairy vetch should not exceed 60% if the residues are to increase N availability. Further study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil. Received: 10 March 1997     

Nitrogen availability to grain sorghum from organic and inorganic sources on sandy and clayey soil surfaces in a greenhouse pot study

In a greenhouse pot study, we examined the availability of N to grain sorghum from organic and inorganic N sources. The treatments were¹⁵N-labeled clover residues, wheat residues, and fertilizer placed on a sandy clay loam and loamy sand soil surface for an 8-week period. Soil aggregates formed under each soil texture were measured after 8 weeks for each treatment. Significantly greater ¹⁵N was taken up and recovered by grain sorghum in sandy clay loam pots compared with loamy sand pots. Greater ¹⁵N recovery was consistently observed with the inorganic source than the organic sources regardless of soil texture or time. Microbial biomass C and N were significantly greater for sandy clay loam soil compared with the loamy sand. Microbial biomass ¹⁵N was also significantly greater in the sandy clay loam treatment compared to the loamy sand. The fertilizer treatment initially had the greatest pool of microbial biomass ¹⁵N but decreased with time. The crop residue treatments generally had less microbial biomass ¹⁵N with time. The crop residues and soil texture had a significant effect on the water-stable aggregates formed after 8 weeks of treatments. Significantly greater water-stable aggregates were formed in the sandy clay loam than the loamy sand. Approximately 20% greater water-stable aggregates were formed under the crop residue treatments compared to the fertilizer only treatment. Soil texture seemed to be one of the most important factors affecting the availability of N from organic or inorganic N sources in these soils.Contribution from the MissouriAgricultural Experiment Station, Journal Series No.12131     

Soil C and N dynamics and maize (Zea may L.) yield as affected by cropping systems and residue management in North-western Pakistan

This paper presents the results of irrigated rotation experiment, conducted in the North West Frontier Province (NWFP), Pakistan, during 1999–2002 to evaluate effects of residues retention, fertilizer N and legumes in crop rotation on yield of maize (Zea mays L.) and soil organic fertility. Chickpea (Cicer arietinum L) and wheat (Triticum aestivum L) were grown in the winters and mungbean (Vigna radiata) and maize in the summers. Immediately after grain harvest, above-ground residues of all crops were either completely removed (−residue), or spread across the plots and incorporated by chisel plough by disc harrow and rotavator (+residue). Fertlizer N rates were nil or 120 kg ha⁻¹ for wheat and nil or 160 kg ha⁻¹ for maize. Our results indicated that post-harvest incorporation of crop residues significantly (p < 0.05) increased the grain and stover yields of maize during both 2000 and 2001. On average, grain yield was increased by 23.7% and stover yield by 26.7% due to residue incorporation. Residue retention also enhanced N uptake by 28.3% in grain and 45.1% in stover of maize. The soil N fertility was improved by 29.2% due to residue retention. The maize grain and stover yields also responded significantly to the previous legume (chickpea) compared with the previous cereal (wheat) treatment. The legume treatment boosted grain yield of maize by 112% and stover yield by 133% with 64.4% increase in soil N fertility. Similarly, fertilizer N applied to previous wheat showed considerable carry over effect on grain (8.9%) and stover (40.7%) yields of the following maize. Application of fertilizer N to current maize substantially increased grain yield of maize by 110%, stover yield by 167% and soil N fertility by 9.8% over the nil N fertilizer treatment. We concluded from these experiments that returning of crop residues, application of fertilizer N and involvement of legumes in crop rotation greatly improves the N economy of the cropping systems and enhances crop productivity through additional N and other benefits in low N soils. The farmers who traditionally remove residues for fodder and fuel will require demonstration of the relative benefits of residues return to soil for sustainable crop productivity.     

Simulating nitrate retention in soils and the effect of catch crop use and rooting pattern under the climatic conditions of Northern Europe

This model analysis of catch crop effects on nitrate retention covered three soil texture classes (sand, loamy sand, sandy loam) and three precipitation regimes in a temperate climate representative of northern Europe (annual precipitation 709–1026 mm) for a period of 43 years. Simulations were made with two catch crops (ryegrass and Brassica) with different rooting depths, and soil N effects in the next spring were analysed to 0.25, 0.75 and 2.0 m depth to represent the catch crop effect on following crops with different rooting depths. Nitrate retained without a catch crop was generally located in deeper soil layers. In the low precipitation regime the overall fraction of nitrate retained in the 0–2.0 m soil profile was 0.23 for the sandy soil, 0.69 for the loamy sand and 0.81 for the sandy loam. Ryegrass reduced leaching losses much less efficiently than Brassica, which depleted nitrate in the 0–0.75 m soil layer more completely, but also in the deeper soil layer, which the ryegrass could not reach. A positive N effect (N_eff, spring mineral N availability after catch crop compared with bare soil) was found in the 0–0.25 m layer (that is shallow rooting depth of a subsequent main crop) in all three soil texture classes, with on average 10 kg N/ha for ryegrass and 34 kg N/ha for Brassica. Considering the whole soil profile (0–2.0 m deep rooting of next crop), a positive N_eff was found in the sand whereas generally a negative N_eff was found in the loamy sand and especially the sandy loam. The simulations showed that for shallow‐rooted crops, catch crop N_eff values were always positive, whereas N_eff for deeper‐rooted crops depended strongly on soil type and annual variations in precipitations. These results are crucial both for farmers crop rotation planning and for design of appropriate catch crop strategies with the aim of protecting the aquatic environment.     

Cassava-legume intercrop: I. Effects of relative planting dates of legumes on cassava productivity

ABSTRACT

Cassava is a long duration crop which grows slowly during the early stages hence incorporation of early maturing crops may improve resource use efficiency and hence crop productivity in cassava-based cropping systems. We conducted field experiments in clay and loamy sand soils, in two consecutive seasons, to assess the response of cassava productivity to relative planting dates of 3 legume species (Bambara groundnut, chickpea and cowpea) in a cassava-legume intercrop. Root yield, determined at 10 months after planting, was greater at 4 weeks after cassava (1.7 kg plant^–1) compared to when legumes were planted same time (1.2 kg plant^–1) and 2 weeks after cassava (0.7 kg plant^–1), and intercropping decreased cassava root yield by 40% (cowpea) and 26% (chickpea) in season I in the clay soils. Neither relative planting dates of legumes nor cropping systems affected root yield in both seasons in the loamy sand soils. Intercropping increased LER in season I by 25% to 65% in clay, and 45% to 101% in loamy sand soils. Although cassava root yield was highest with a 4 week-delay in sowing the legumes and LER greatest in cassava-cowpea intercrop, we propose further studies before making any definite recommendations.     

Different crop rotation systems as drivers of change in soil bacterial community structure and yield of rice, <Emphasis Type="Italic">Oryza sativa</Emphasis>

Intensive cropping, especially of rice, is considered to contribute to negative effects not only on soil chemical and biological properties but also on long-term grain yield. Appropriate crop rotation is often practiced as an alternative strategy to overcome the negative side effects of intensive cropping. Although soil microbial diversity and community structure have been shown to respond differently to altered agricultural management practices, little is known about possible links between crop rotation and grain yield on bacterial communities in rice paddy soil. In this study, we investigated the impact of specific rotational crops and compared it with intensive rice cultivation. The main crop rice (Oryza sativa) was rotated with maize (Zea mays) and mungbean (Phaseolus aureus) in different combinations in a system cultivating three crops per year. Soil bacterial communities were studied in two different cropping periods using pyrosequencing of the variable V4 region of the 16S rRNA. Our results showed that rotation with alternative crops increased rice yield by 24–46% depending on rotation structure and that bacterial community structure was altered in the presence of mungbean and/or maize compared to that in rice monoculture. In the crop rotation systems, composition, abundance, and diversity of soil bacterial communities were significantly different and higher than those in rice monoculture. Our results show that effects of crop rotation relate to changes in soil bacterial community structure suggesting that appropriate crop rotations provide a feasible practice to maintain the equilibrium in soil microbial environment for sustainable rice cultivation.     

Contributions from carbon and nitrogen in roots to closing the yield gap between conventional and organic cropping systems

This study investigates the effect of different crop rotation systems on carbon (C) and nitrogen (N) in root biomass as well as on soil organic carbon (SOC ). Soils under spring barley and spring barley/pea mixture were sampled both in organic and conventional crop rotations. The amounts of root biomass and SOC in fine (250–253 μ m), medium (425–250 μ m) and coarse (>425 μ m) soil particulate organic matter (POM ) were determined. Grain dry matter (DM ) and the amount of N in harvested grain were also quantified. Organic systems with varying use of manure and catch crops had lower spring barley grain DM yield compared to those in conventional systems, whereas barley/pea showed no differences. The largest benefits were observed for grain N yields and grain DM yields for spring barley, where grain N yield was positively correlated with root N. The inclusion of catch crops in organic rotations resulted in higher root N and SOC (g C/m²) in fine POM in soils under barley/pea. Our results suggest that manure application and inclusion of catch crops improve crop N supply and reduce the yield gap between conventional and organic rotations. The observed positive correlation between root N and grain N imply that management practices aimed at increasing grain N could also increase root N and thus enhance N supply for subsequent crops.     

Yield and nitrogen response of lowland rice (Oryza sativa L.) to Sesbania rostrata and Aeschynomene afraspera green manure in different marginally productive soils in the Philippines

Organic-N fertilizers in the form of flood-tolerant, leguminous, stem-nodulating Sesbania rostrata and Aeschynomene afraspera may be useful alternatives to resource-poor rice farmers if applied as green manure. Therefore, the accumulation of N by these green manure species and their effect on the performance and yield of wetland rice (IR 64) was examined at four different sites in Luzon, Philippines. Soils deficient in N, P, and K were selected and compared with the fertile Maahas clay of the International Rice Research Institute (IRRI) at Los Baños. The green manure plants were grown under flooded conditions for 49 days in the wet season of 1987, chopped, and then ploughed in before transplanting rice seedlings. In a second experiment, the effect of S. rostrata green manure was studied under rainfed conditions. All green manure treatments were compared to an urea treatment (60 kg N ha^–1) and an untreated control. Both legumes developed well, even on the marginally productive soils. S. rostrata accumulated up to 190 kg N ha^–1 and A. afraspera even accumulated 196 kg N ha^–1 in the shoots. In all treatments, green manure increased grain yield significantly (P=0.05) over the untreated control, by 1.3–1.7 Mg ha^–1. The yields were comparable to those obtained with 60 kg N ha^–1 of urea fertilizer. S. rostrata caused the highest grain yield, of 6.5 Mg ha^–1 on the Maahas clay soil of IRRI. The apparent release of exchangeable NH ₄ ⁺ -N in the soils after green manuring and the rice grain yield response showed that both green manure species may provide sufficient available N throughout the development of IR 64 in the wet season. In the rainfed marginal soil site, green manure with S. rostrata produced even higher rice grain yields than urea. Green manure therefore seems particularly attractive for poor farmers on marginally productive soils, at least as a temporary strategy to improve yield and yield sustainability.