RESUMO
The growth of cover crops can contribute to the increase in phosphorus content at depth by root decomposition. The aim of this work was to verify the effect of cover crops on soil phosphorus availability and use by successive plants, and the accumulation of soil P in a no-tillage system conducted for 14 years. This research was carried out during the 2016/2017 and 2017/2018 crop seasons, whose treatments have been installed and maintained since 2003. The experimental design was a randomized block design, and the plots consisted of spring crops: pearl millet, forage sorghum, sunn hemp, and additionally, a fallow/chiseling area. The evaluation of available P was determined by P fractionation. In general, in the two years of evaluation, the accumulation of P in the shoot dry matter was higher in sunn hemp growth, on average 25% higher than pearl millet in 2016 and 40% higher than sorghum in 2017. The highest contents of labile inorganic P were in the sorghum-soybean and fallow/chiseling-soybean successions, with values higher than 50 mg kg-1 of P in the 0-0.1 m soil layer. However, in the other layers analyzed, the cover crops obtained higher availability of labile inorganic P. The systems using cover crops recovered 100% of the P fertilized in soybean.
RESUMO
Phosphorus (P) availability is important for metabolic process, tillering and formation of a vigorous root system in sugarcane, but sugarcane varieties differ in P uptake efficiency. This study evaluated the enzymatic, nutritional, and biometric parameters of two sugarcane varieties under two conditions of P availability by monitoring the initial development of plants grown in nutrient solution. The experiment was performed using randomized complete block design (RCBD) with five replicates and included two varieties, RB966928 (high nutritional requirements) and RB867515 (low nutritional requirements), and two concentrations of P in the nutrient solution: low (2 mg L-1) and suitable (16 mg L-1). Carbohydrate concentrations and partitioning, leaf nutrient concentrations, enzymatic activity, and shoot and root biometric parameters were analyzed. Regardless of sugarcane variety and the part of the plant, reducing sugar were approximately 32.5% higher in RB867515 and 38.5% higher in RB966928 under suitable P compared with low P. Sucrose concentrations were significantly higher in both varieties under suitable P than in low P. According to PCA, the relationship between reducing sugars and sucrose was closer in RB966928 than in RB867515. Under low P, soluble protein content decreased, and the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) and the concentrations of hydrogen peroxide (H2O2), and malondialdehyde (MDA). The variety RB966928 under suitable P appears to have a high capacity for proline (120%) upregulation under abiotic stress compared with RB867515 (54%), and thus higher biomass accumulation of this RB966928 variety; however, RB867515 had superior results compared to RB966928 under low P. Suitable P increased leaf concentrations of N, P, Mg, B, and Mg and decreased leaf Zn content. Root and shoot dry matter, root length, plant height, and root and stalk diameter increased by suitable P. Regardless of variety, both nutritional and biometric parameters were directly influenced by P levels, including sugarcane yield. In relation of sugarcane dry matter, RB966928 was less sensitive to low P levels and more responsive to P supply than RB867515 and thus may be more suitable for environments in which P is limiting.