Effect of cotton-cowpea intercropping on C and N mineralisation patterns of residue mixtures and soil

Carbon and nitrogen mineralisation potential of mixed cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) crop residues produced under intercropping, as well as a reddish-brown soil classified by FAO as Ferralic Cambisol previously under intercrops, were studied over a 10-week incubation period under controlled conditions (25 degrees C and moisture content of 70% field capacity, 125 mm) in the laboratory. Treatments consisted of cotton residues ( 100 : 0), cowpea residues ( 0 : 100), and cotton-cowpea residues ( 50 : 50, 70 : 30, and 30 : 70). These ratios were based on yields obtained in different cotton-cowpea intercrop treatments from a field study. Cowpea residues ( 0 : 100) released the highest amount of mineral N of 36.4 mg/kg soil, and cotton residues ( 100 : 0) least, 19.2 mg/kg soil, while the other mixtures were in between. All treatments except for cowpea residues ( 0 : 100) and the 30 : 70 mixture showed immobilisation of soil N during the first 2 weeks of incubation. The trend for C mineralisation was similar to that of N, and cowpea residues ( 0 : 100) released the highest amount, 492 mg C/kg soil, while cotton residues ( 100 : 0) recorded the least, 315 mg C/kg soil. The C mineralisation patterns of cowpea residues ( 0 : 100) and 30 : 70 treatments were exponential and were well described by the equation: C = C-E(1 - e(-kt)) where C-E is exponentially mineralisable C fraction, k is the rate constant, and t is time in days. The mineralisation patterns for other treatments were sigmoidal and were well described by the equation: C = C-S/1+e(-(t t0/k)) where CS is sigmoidally mineralisable C fraction; t(0) is time in days required for complete mineralisation of CS, while k is rate constant. The amount of N released from soil previously under cotton-cowpea intercrops and sole crops was approximately one-third of the amount released when the residues were incorporated. The highest amount of N released (12.2 mg/kg soil) was from soil previously under sole cowpea, while soil from the 1 : 1 cotton-cowpea intercrop released 9.9 mg/kg soil and soil from sole cotton released 5.9 mg/kg soil. There was no significant effect ( P > 0.05) of previous crop on C mineralisation patterns of the soil. Mixtures slow down N losses and increase nutrient use efficiency of legume residues, especially in the short-term. When cotton is grown as a sole crop, starter N to offset negative effects of initial N-immobilisation at the start of season is required. A better understanding of controlling parameters of decomposition can make it possible to predict C and N mineralisation patterns in mixtures. Reduced C mineralisation in cotton-cowpea mixtures may result in more C sequestration and, hence, SOM build-up and improved sustainability in the long term in intercropping systems.