Weeds – friend or foe? The role of weed composition on stover nutrient recycling efficiency

In the upland soils of Northeast Thailand, most farmers can grow only one crop per year and soils are left fallow until the next crop, leading to the establishment of a native weed population. Additionally, these areas are often very sandy thus posing a high risk of nitrogen leaching during late or erratic and early rains occurring during the fallow period. Hence the objective of this study was to assess the impact of weeds and weed composition, i.e. native weed mixture, grass or legume/broadleaf dominated weed fallows over the dry season on nutrient recycling of groundnut (Arachis hypogaea L.) stover, i.e. mineral N and microbial biomass N dynamics, 15N pre-crop residue recovery and subsequent effects on maize yield.Maize establishment was slowest (P < 0.05) where grass and mixed weed grew during the fallow phase and maize dry weight was initially highest (P < 0.05) in plots where no weeds were allowed to grow during the dry season. The negative effect of grass dominated weeds was due to microbial immobilization of soil mineral N in the presence of the low quality (higher C:N ratio and ADF fibre content) grass weeds. In the second maize crop there were some indications of release of immobilized microbial N. Negative effects of grass dominated weed fallows could be overcome by addition of split doses of N fertilizer. In contrast legume/broadleaf dominated weed populations led to less microbial immobilization of mineral N and resulted in faster net release of mineral N (P < 0.05). As a consequence, maize yields in the legume/broadleaf weed treatment (5428 kg ha−1) were higher than in the grass (2899 kg ha−1), mixed weed (3120 kg ha−1) or no weed (3552 kg ha−1) fallows over two cropping cycles. 15N balance results indicated that mixed weeds retained more groundnut residue N in the soil organic pool than the treatment without weeds during the fallow phase although the effect was not significant at P < 0.05.Identifying management options that result in a dominance of broadleaf weed composition will be crucial if weeds growing during the dry season fallow period are to improve short-term nutrient recycling. Recycling grass dominated weed biomass will require judicious fertilizer amendments or incorporation well before maize planting to avoid competition for mineral N by the soil microbial biomass.