Nitrogen and phosphorus assimilation, mobilization and partitioning in rainfed chickpea (Cicer arietinum L)

A field experiment was conducted to assess the sources of N (N-2 fixation, soil and fertilizer), N and P partitioning and mobilization in rainfed chickpea at various growth stages using a N-15 technique. The A-value method, involving the application of 20 and 100 kg N ha(-1) of N-15 labelled ammonium sulphate to chickpea and wheat as a reference crop respectively, was used to determine the relative contribution of N for these sources. Dry matter, N and P content in shoots, roots and nodules declined rapidly after flowering, whereas, those of pods showed considerable increases. Soil water content - measured by the neutron scattering technique - decreased severely after flowering. Furthermore, the crop was exposed to heat stress at pod filling which caused a decline in N, fixation and affected the distribution of both N and P in the plant. The peak of N-2 fixation occurred between bud flower initiation and maximum flowering. It was 3 kg N ha(-1) day(-1) and 41 kg N ha(-1) per growth period. N-2-fixation rate peaked at flowering and declined sharply thereafter during pod filling. Such decline was a result of changes in source-sink relationships, resulting in a greater supply of carbohydrate and phosphorus to the developing pods and a reduced supply to nodules which senescence rapidly following flowering. There was net mobilization of N and P from shoots and roots to developing pods after flowering which accounted for 81% and 70% of N and P in pods respectively. At physiological maturity, chickpea had accumulated 103 kg N ha(-1), 60% of which was derived from fixation, 35% from soil and 5% from fertilizer. After seed removal, there was only a small residual return of fixed N to soil. Methods to improve N-2 fixation in rainfed grown chickpeas are discussed.