Photodegradation and microbial decomposition of soybean and maize crop residues before and after harvest

The role of sunlight and its interaction with microorganisms in crop residue decomposition is largely unknown in agroecosystems. We evaluated soybean and maize residue decomposition before harvest, when plants were senescent and standing, and after harvest, during winter fallow, when residues remained on the soil surface. We hypothesized that (1) sunlight dominates decomposition before harvest, whereas microbes become more relevant after harvest, and (2) soybean residues decompose faster than maize due to higher susceptibility to sunlight-driven decay before harvest and greater nutrient content, enhancing microbial decomposition after harvest. We conducted a field experiment in the Argentine Pampas, manipulating sunlight and microbes with filters and a biocide, respectively. Contrary to our expectation, decomposition before harvest was significantly accelerated by both sunlight and microbes additively, resulting in carbon losses of approximately 141 kg C ha⁻¹ in maize and 108 kg C ha⁻¹ in soybean leaf residues. Sunlight alone accounted for 15–24 % of total leaf mass loss, while microbes contributed 30–54 %. The results indicated that sunlight acted through photodegradation rather than photofacilitation of microbial decomposition. Before harvest, decomposition was twice as fast as after harvest, with no effect of sunlight or microbes. Soybean residues decomposed faster than maize due to greater susceptibility to sunlight (canopy structure and lignin) and enhanced microbial activity before harvest, and lower mechanical resistance after harvest. These findings highlight that both sunlight and microbes substantially contribute to residue turnover, particularly of standing dead crops before harvest, a critical yet often overlooked stage in cropland carbon dynamics.