Changed Stoichiometric Ratios of Carbon, Nitrogen, and Phosphorus in Runoff Water from Maize Upland Fields by Rice Straw Cover

Abstract

Soil and nutrient loss through runoff water from upland fields is a critical non-point pollution pathway in agricultural areas. Surface cover with crop residue is commonly recommended to reduce soil and nutrient loss; however, there is a high possibility of increased runoff loss of dissolved organic C (DOC) through decomposition of crop residue though relevant studies are lacking. The changed stoichiometric ratios of carbon (C), nitrogen (N), and phosphorus (P) such as C/N and C/P of runoff water by increased DOC loss can be indicators of the impact of surface cover on water quality. In this study, the effects of rice straw cover on C/N and C/P of rainfall-derived runoff water (totally six events) from upland maize fields were investigated. The C/N and C/P in the sediment-bound form were not affected by straw cover throughout the six rainfall events, across the control and covered plots. The C/N and C/P in dissolved forms varied from 0.9 to 3.3 (1.8 ± 0.4) and from 4.6 to 43.4 (12.1 ± 2.8) for control plot and from 1.8 to 37.1 (8.7 ± 0.8) and from 6.1 to 126.2 (44.4 ± 6.5) for covered plots, respectively. Surface cover increased (p < 0.05) the mean C/N (by 4.8 fold) and C/P (3.7 fold) in the dissolved forms across the six events, and increased C/N and C/P were evident in the late period of long-lasting rainfall events when surface cover lost its capacity to reduce runoff. Therefore, surface cover of upland fields with crop residue may increase C/N and C/P of runoff water, and thus these stoichiometric ratios in water bodies surrounding the fields may be used as indicators of the influence of upland fields covered with crop residue on water pollution particularly in the late period of continuous rainfall. Rice straw surface cover of maize upland fields increased C/N and C/P in dissolved forms of runoff water from the fields due to increased export of dissolved C through straw decomposition.