Global Theme on Agroecosystems
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Global Theme on Agroecosystems
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Long-term Effects of Legume-based Cropping System Rotation in Vertisol In semi-arid India, a large area of Vertisols are left fallow during the rainy season and cropped during post-rainy season with stored soil moisture. The reasons for fallowing are different in low and high rainfall area. In areas of low (500-750 mm) and erratic rainfall, rainy season cropping is risky despite good moisture holding capacity of soil. On the other hand in wetter areas (> 750 mm) this soil becomes too sticky and difficult to manage and crops may suffer from water logging. Early research at ICRISAT concentrated on the later soil types and developed "Vertisol technology" by which farmers can not only crop during rainy and post-rainy season but also reduce soil erosion during rainy season. However, the research also pointed out that adequate plant nutrient supply is the major constraint for the success of this technology since most of these soils are deficient in essential plant nutrients. Cereals and other non-legumes gave large response to applied N followed by P. In spite of getting good response farmers were reluctant to apply inorganic fertilizers because of the risk due to erratic type of rainfall situation occurring in these areas. Alternate and less risky source of nutrients was considered a priority research area. To start with the concentration was given to N supply through legumes. A long-term experiment that examines the yields and residual effects of various combinations of crops to assess optimum combinations of legumes with sorghum for maintaining productivity was established on a Vertisol at ICRISAT. The three objectives of this study were 1) to quantify the benefits of grain legumes to non-legumes in the cropping system rotations; 2) to monitor long-term changes in soil quality for sustainable crop production; and 3) to identify improved and sustainable cropping system options for crop intensification and enhancing crop productivity. The field experiment with 10 different cropping system rotation was established in 1983. The treatment details are given in Table 1. The design is split plot with cropping systems as main plots and levels of N fertilizer application as subplots. Among the cropping systems, five are continuous and another five are rotated in a two-year cycle. All the cropping systems, except the rainy season fallow system, have two crops, grown either as intercrops dry-sown seven to ten days before the onset of the south-west monsoon and the second crop immediately after the harvest of the first crop in each year. All the five rotations had their mirror images so that in any given year both phases of rotations were grown. Main plot size was 12 x 12 m and each plot was divided into four subplots receiving 4 levels of nitrogen fertilizer only to non-legume crops as urea.
Sub-plots: Levels of nitrogen fertilization ( to the non-legumes only) N o = 0 kg N ha -1 No. of Replications: 3
Rainy-season sorghum
From the long-term sustainability point of view, the pigeonpea-based systems were not only superior to the chickpea based as well as continuous non-legume (sorghum+ safflower) system but also these systems showed upward increase in grain yield of the following sorghum from 1989 onwards (Fig. 2).
Figure 2: Three year moving average of sorghum grain yield as affected by preceding legume based cropping system compared to a non-legume cropping system without N application during 1984-94. The uptake of N by sorghum following C/PP was highest in all years from 1984 onwards. The P and K uptake also followed similar trend. The N uptake by sorghum following 100% legume (C/PP) system was 28 kg N ha -1 higher than that of sorghum following a non-legume (S+SF) the uptake of which was only 20 kg N ha -1 . The uptake of sorghum following S/PP was 15 kg N ha -1 more while that of following S+CP system was 7 kg N ha -1 more than sorghum after the S+SF system (Table 2).
Even though the soil analysis from different plots after 16 years is not completed the organic carbon levels are significantly different in different cropping systems only in 0-15 cm layer. The O.C. content in other layers upto 150 cm are not affected either by cropping systems or by N levels. All the pigeonpea based cropping system had significantly more O.C. when compared to other systems. The soil total N analyzed after 10 years also revealed a build up of soil N nearly 125 m g/g soil in continuous S/PP – S/PP system while other pigeonpea based system also recorded > 600 m g/g soil when compared to the original N level of 550 m g/g soil. There are several possible reasons for the increased yield of non-leguminous plants after legumes. Increased nitrogen availability is considered one of the important factors responsible for the beneficial effect of the legume on the following non-legume crop. The improved grain yield of the non-legume has also been attributed to other factors, namely decreased pest and disease pressure, reduction of allelopathic effects from cereal crop residues, and improvement in soil structure and water-holding capacity. However, the legume benefits on sorghum observed in this trial were caused mainly by increased N availability. Sorghum grain yield was different at low N fertilizer levels where N availability was limiting, but the differences were not significant at 120 kg N. As intensive plant protection measures were followed during crop growth, any benefits from reduced pests and diseases were ruled out. There was another indication that improved N availability was the reason for higher sorghum yields after legumes, and this was the increased soil mineral and mineralizable N content in the years following S/PP and C/PP. The additional amounts of mineral plus mineralizable N after S+CP, S/PP and C/PP on average were 4, 32 and 30 kg N ha -1 respectively which agreed in order of magnitude with the increase in sorghum N uptake following the legumes. An increase in total soil N under pigeonpea-based rotations occurred despite removal of all standing crop residues and increased N uptake by sorghum (47 kg N ha -1 in the C/PP±S+SF system and 35 kg N ha -1 in the S/PP±S+SF system compared with 27 kg N ha -1 in the S+CP±S+SF system). In conclusion, medium-duration pigeonpea increased the total-N of the soil and also increased N availability for succeeding sorghum, thereby supporting sustainable sorghum yields averaging 2.7 t ha -1 in a rotation without mineral N inputs. As discussed above a low N harvest index and considerable in situ N recycling through leaf fall during its growth were the reasons for higher N inputs by pigeonpea than by chickpea. Even though medium-duration pigeonpea appeared to be ideally suited for different cropping systems in semi-arid tropical India, not only as a supplier of protein-rich grains but also a means of N supply in low-input dryland agriculture, the two-year narrow rotations showed decreasing pigeonpea yields due to a resistant insect, Helicoverpa, and a slight build up of pigeonpea wilt and nematodes. Thus, the optimum rotation may not be one of growing pigeonpea as often as every second year; instead it may be better included in a wider rotation system. However, in terms of sorghum productivity, it is an improved but also a sustainable system. For more information please contact:
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grain yield production was sustained at ~ 2.7 t ha -1 over 12 years with a continuous sorghum/pigeonpea intercrop system. From a cowpea/pigeonpea intercrop system, succeeding sorghum got benefited by about 40 kg N ha -1 (fertilizer equivalent) annually as the grain yield was around 3.3 t ha -1 without nitrogen fertilizer application (Fig. 1). The traditional fallow-sorghum produced 1.1 t ha -1 grain yields whereas fallow-chickpea-fallow sorghum system increased the grain yield of sorghum by 0.3 t ha -1 . The continuous mungbean-sorghum system also produced 1.4 t ha -1 of sorghum grain yield. 
