Degradation of agricultural waste is dependent on chemical fertilizers in long-term paddy-dry rotation field

Abstract

The practice of returning straw to agricultural fields is a globally employed technique. Such agricultural fields also receive a significant amount of nitrogen (N) and phosphorus (P) fertilizers, because these two macronutrients are essential for plant growth and development. However, the consequences of such macronutrients input on straw decomposition, soil dissolved organic matter (DOM), key microbes, and lignocellulolytic enzymes are still unclear. In a similar aim, we designed a long-term straw returning study without and with different N and P nutrient supplementation: CK (N₀P₀), T1 (N₁₂₀P₀), T2 (N₁₂₀P₆₀), T3 (N₁₂₀P₉₀), T4 (N₁₂₀P₁₂₀), T5 (N₀P₉₀), T6 (N₆₀P₉₀), and T7 (N₁₈₀P₉₀), and evaluated their impact on rice and oilseed rape yield, soil DOM, enzymes, lignocellulose content, microbial diversity, and composition. We found straw returning improved overall yield in all treatments and T7 showed the highest yield for oilseed rape (30.31-38.87 g/plant) and rice (9.14-9.91 t/ha) during five-years of study. The fertilizer application showed a significant impact on soil physicochemical properties, such as water holding capacity and soil porosity decreased, and bulk density increased in fertilized treatments, as compared to CK. Similarly, significantly low OM, cellulose, hemicellulose, and lignin content were found in T7, T4, T3, and T2, while high values were found in CK and T5, respectively. The fluorescence excitation-emission matrix spectra of DOM of different treatments revealed that T3, T7, T4, and T6 showed high peak M (microbial by-products), peak A and peak C (humic acid-like) as compared to others. The microbial composition was also distinctive in each treatment and a high relative abundance of Chloroflexi, Actinobacteriota, Ascomycota, and Basidiomycota were found in T2 and T3 treatments, respectively. These findings indicate that the decomposition of straw in the agricultural field was dependent on nutrients input, which facilitated key microbial growth and impacted positively on lignocellulolytic enzymes, which further aided the breakdown of all components of straw in the field efficiently. On the other hand, high input of chemical based fertilizers to soil can lead to several environmental issues, such as nutrient imbalance, nutrient runoff, soil pH change and changes in microbial activities. Keeping that in consideration, we recommend moderate fertilizer dosage (N₁₂₀P₉₀) in such fields to achieve higher decomposition of crop straw with a small yield compromise.

Keywords: Crop straw; Enzymes; Fluorescence excitation-emission matrix; Organic matter; Paddy field.