Building Organic Matter of Long-Term Sugarcane Soils in a Temperate Environment.

Mineral soils with a history of sugarcane monoculture cropping contain less soil organic matter (-35%), and plant macro- and micronutrients, including N (-20%), K (-26%), S (-7%), Ca (-8%), B (-33%), Zn (-88%), Mn (-29%), and Cu (-26%), than adjacent non-cultivated soils. Harvesting sugarcane ‘green’ deposits up to 11.0 Mg/ha of dry tops and leaf material annually, and could build soil organic matter and plant available nutrients over time. But, residue decomposition is slow and delays emergence of new shoots. Burning residue is recommended but results in a loss of C, N, and S. Repositioning the residue into wheel furrows is partially successful depending on post-harvest field conditions. Treating the residue in the field with urea, molasses, and sulfuric acid, has minimal effect on decomposition, possibly indicating abiotic control on soil activity. Therefore, the objective was to observe the soil’s response to summer (32°C), fall/spring (23°C), and winter (9°C) temperatures, soil moisture levels of 0.13, 0.23, and 0.28 g water /g soil, and sugarcane residue additions (0.5% by weight). Headspace CO₂ was quantified by gas chromatography and used to calculate the amount of soil and residue C mineralized over 155 d. Soil C mineralization increased with temperature with means of 160, 540, and 880 ug C /g soil at 9°C, 23°C, and 32°C, respectively. Water content exhibited a lesser effect, with 400, 510, and 580 ug soil C mineralized /g soil at 0.13, 0.23, and 0.28 g/g, respectively. The highest mineralization rates for residue-amended soils occurred between 4-8 d, regardless of incubation conditions. The results suggest sugarcane residue decomposes in soil rapidly at temperatures ≥ 23°C and soil moisture ≥ 0.23 g/g. Brushing the residue into the wheel furrow, and waiting on soils to warm, may improve C and nutrient recycling in the soil without reducing crop yields.