Mixing groundnut residues and rice straw to improve rice yield and N use efficiency

Groundnut as a pre-rice crop is usually harvested 1-2 months before rice transplanting. During this lag phase much of N in groundnut residues could be lost due to rapid N mineralization. Mixing of abundantly available rice straw with groundnut residues may be a means for reducing N and improve subsequent crop yields. The objectives of this experiment were to investigate the effect of mixing groundnut residues and rice straw in different proportions on (a) growth and yield of succeeding rice, (b) groundnut residue N use efficiency and (c) N lost (¹⁵N balance) from the plant-soil system and fate of residue N in soil fractions. The experiment consisted of six treatments: (i) control (no residues), (ii) NPK (at recommended rate, 38kgNha⁻¹), (iii) groundnut residues 5Mgha⁻¹ (120kgNha⁻¹), (iv) rice straw 5Mgha⁻¹ (25kgNha⁻¹), (v) 1:0.5 mixed (groundnut residues 5Mg: rice straw 2.5Mgha⁻¹), and (vi) 1:1 mixed (groundnut residues 5Mg: rice straw 5Mgha⁻¹). After rice transplanting, samples of the lowland rice cultivar KDML 105 were periodically collected to determine growth and nutrient uptake. At final harvest, dry weight, nutrient contents and ¹⁵N recovery of labeled groundnut residues were evaluated. Significant effects of residue management treatments on rice growth and nutrient uptake dynamics were observed from 30 days after transplanting onwards. At final harvest, the highest grain yield and N content of rice were obtained in the 1:1 (3.7Mgha⁻¹) and 1:0.5 mixed treatments followed by sole groundnut residues>rice straw>NPK>control (2.8Mgha⁻¹) treatments. There was a significant relationship between mineral N content in soil before transplanting and rice yield at harvest. Mixed residue treatments had significantly (p <0.1) higher groundnut-derived ¹⁵N recoveries (13%) in rice than in the sole groundnut treatment (10%) but they were lower than the recovery of labeled mineral N fertilizer in the NPK treatment (29%). The highest ¹⁵N recoveries in all treatments were observed in the topsoil (0-15cm) where over 80% of ¹⁵N was located. Systems N unaccounted for from groundnut residues (32%) were similar to fertilizer N losses (30%), but they were lowest in the 1:0.5 mixed treatment (15%). The highest δ ¹³C value occurred in residue treatments and ¹³C was positively related to N uptake while negatively related to internal N use efficiency. The result from our study demonstrated that mixing groundnut residues and rice straw could delay N release during the pre-rice lag phase leading to an improved synchrony in N demand/supply and increased growth and yield of the succeeding rice and reduced N losses from the soil-plant system.