Analysis of Competition and Phosphorus Response in Maize/Soybean and Maize/Rice Intercrops in Relation to Soil Phosphorus Availability in Different Environments

Rajbhandari, Neeranjan P.
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While the effect of nitrogen on intercrops has been extensively studied, little information is available on P effects. There is a dearth of information on how intercrops respond to varying levels of soil P availability leading to more efficiency. Field experiments were therefore conducted at University of Hawaii Experiment Stations in three environments to evaluate the productivity of intercrops, leaf properties, and root dry matter in relation to P in the soil solution. Intercrops of maize with soybean or rice were established at ten levels of soil solution P and evaluated against sole crop checks for grain yield, dry matter production, P uptake and P use efficiency to determine whether the increased productivity of the mixture was only due to increased uptake of resources or efficient conversion to dry matter or grain yield by intercrop components under competition. Differences between environments were large relative to the effects of P and intercropping systems. The response of grain and total dry matter yield to P (9Y/aP) was proportional to the inverse of P level for both maize and soybean in each environments. The response of the intercrop maize to P, was similar to that of the sole crop. The presence of soybean with maize had little effect on the performance of intercrop maize, however soybean yields were significantly reduced. The response of intercrop soybean to environment and P level was different than its sole crop. Grain and dry matter yield of sole crop soybean increased with increased P availability whereas intercrop soybean yield decreased. The magnitude of the increase or decrease (slope) depended on environment and the sign of the slope changed by intercropping. Higher maize yields across environments and P levels were associated with reduced growth of intercrop soybean. Intercrop advantage, as measured by the Land Equivalent Ratio, and the competitiveness of soybean decreased as P availability increased. The increased competitiveness of intercrop maize at high P levels was correlated with a reduction in yield of intercrop soybean. The advantage due to intercropping was maximum under low soil P availability under a wide range of environmental conditions. Growth of intercrop maize was no different than the sole crop for their leaf properties and P uptake, but was profoundly affected by environment and P availability in the soil. Soybean leaf properties, leaf tissue P concentration and P uptake were affected by environment, P level and intercrop system and their interactions. Phosphorus uptake increased as the availability increased irrespective of the environment and cropping system. Phosphorus use efficiency, measured as the grain yield or dry matter per unit of P uptake, decreased with increased P availability. Taken together, the intercrops extracted more P than sole crop maize. P use efficiency was reduced by intercropping. Total root biomass (dry weight) in the surface layer of the intercrops was higher than in the sole crops, with the difference changing according to P levels and year. An estimate of LER based on root dry weight was within the range calculated using above-ground dry matter or grain yield. The increased productivity of intercrops was associated with increased P uptake. In low-input subsistence agriculture, accelerated P mining — the faster removal of limited soil P — may cause the intercrop systems to be less sustainable.
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