Qualidade e quantidade de resíduos culturais modulam a estabilização de carbono e nitrogênio no solo

Abstract

Crop residues left on the soil surface play an important role in controlling erosion, nutrient cycling and soil organic matter (SOM) formation. The diversification of cropping systems is a strategy that allows the input of high quantities of crop residues with varied chemical composition. In view of this, the crop residue addition with distinct chemical qualities (chemical characteristics) and quantities can modify the decomposition dynamics of residues and carbon (C) and nitrogen (N) accumulation in the soil. The objective of this study was to understand how the chemical quality and quantity of residues added to the soil surface affect the C and N stabilization in the soil. For this, a field experiment was carried out, in the experimental area of the Department of Soils at Federal University of Santa Maria, in Typic Paleudalf, during 420 days. Crop residues of wheat and vetch (winter) and soybean and sorghum (summer) labeled with 13C and 15N, which were allocated on the soil surface inside polyvinyl chloride (PVC) cylinders (Ф 7.5 and 14 cm high), inserted at 10 cm in the soil, simulating a succession of cultures. The crop residues were applied in quantities of 3 and 6 Mg ha-1 and was used a control treatment, soil without residue. The 13C and 15N remaining, whole soil 13C and 15N and 13C and 15N stabilized in the physical fractions of SOM - particulate organic matter (POM) and mineral-associated organic matter (MOAM) were determined at 28, 140 and 280 days. The amount of 13C and 15N remaining in high quality residues (vetch and soybean) was lower than in low quality residues (wheat and sorghum), regardless of the quantity of residues added. High quality residues contributed more to soil 13C than low quality residues. However, in terms of 13C stabilization efficiency in the MOAM fraction, the low quality residues (wheat) accumulated more 13C than the other residues. High quality residues accumulated more 15N in the soil than low quality residues, although it was proportionally less than added. Regardless of the crop residue quality, the 13C and 15N accumulation in the soil increased with increasing quantity of crop residue. The results obtained confirm that higher chemical quality residues present greater decomposition and accumulation of C and N in the soil. The quantity of crop residues does not affect decomposition, but the addition of more residues leads to a greater C and N accumulation in the soil. High quality residues do not show greater efficiency in stabilizing C in the soil compared to low quality residues.