Abstract
The area under mechanized sugarcane (Saccharum spp.) harvesting is expanding in Brazil, increasing the return of trash to the soil. The main questions regarding this management are: (i) after adopting unburned mechanical harvesting, how long will it take to observe decreases in fertilizer requirements, (ii) what will be the magnitude of this decrease and, (iii) the impact in the short run of removing trash for energy purposes in the nutrient cycling? This study aimed to build an N prediction model for long term assessment of the contribution of sugarcane crop residues to sugarcane nutrition and to evaluate the cycling of other nutrients derived from crop residues. Keeping crop residues over the soil will increase soil N stock and N recovery by sugarcane, reaching equilibrium after 40 years with recovery of approximately 40 kg ha-1 year-1 of N. Removing trash for energy production will decrease the potential reduction in N fertilizer requirement. Of the total nutrients in the trash, 75 % of the K2O (81 kg ha-1 year-1) and 50 % of the N (31 kg ha-1 year-1) are in the tops, indicating the importance of maintaining tops in the soil to sustain soil fertility. Because the input data employed in the simulations are representative of the conditions in Southeast Brazil, these results might not be definitive for situations not represented in the experiments used in the study, but the model produced is useful to forecast changes that occur in the soil under different trash management.
Highlights
In order to simulate the amount of trash-N produced in each crop cycle and the trash-N recovery by sugarcane, the following procedures were adopted: Trash-N produced N from previous crop residues (N-PCR): the amount of N incorporated into the soil after soil preparation prior to sugarcane planting every six years was assumed as 80 kg ha–1 of N (Franco et al, 2007)
Recovery of N from crop residues From the initial amount of N contained in previous crop residues (N-PCR, 80 kg ha–1 N), which were incorporated to the soil before the new cycle began, 25.6 kg ha–1 was recovered by sugarcane in a six year period (Table 3)
The accumulated N from trash left after harvest within the current cycle (RN-PHT per cycle) accounted for 40 kg ha–1 N, yielding 65.5 kg ha–1 N recovered from sugarcane residues in the first cycle
Summary
Sugarcane (Saccharum spp.) trash consists of dry leaves, tops and variable amounts of cane stalks, varying from 10 to 20 t ha–1 of dry matter, presents high C to N ratio (80 to 110:1) and contains 30 to 80 kg ha–1 of N (Basanta et al, 2003; Fortes et al, 2012; Gava et al, 2003; Robertson and Thorburn, 2007a; Thorburn et al, 2005; Trivelin et al, 1995; Vitti et al, 2011). The high C/N ratio of trash results in significant N immobilization due to increased microbiological activity caused by energy input into the soil, leading to slow rates of N release in the short term (Meier et al, 2006). Since most of the trash-derived N stays in the soil N pool (Meier et al, 2006; Fortes et al, 2013), the differences observed can be explained, at least in part, by variations on weather conditions favoring plant growth and uptake of trash-N mineralized
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