Liberação de fósforo e potássio durante a decomposição de palha de cana-de-açúcar sob diferentes taxas de remoção da palha

Abstract

The mechanized harvesting of sugarcane (Saccharum officinarum L.) results in large amounts of straw deposited on the soil surface, arousing the interest of industries in its use for the production of bioethanol and electricity generation. However, the maintenance of straw on the soil surface has numerous benefits, whether physical, chemical, or biological. Thus, the partial removal of straw appears as an alternative. However, when partially removing a straw from the soil surface, mulches of different amounts are formed which can change the rates of decomposition and release of phosphorus (P) and potassium (K) from the straw that remains on the soil surface. Thus, the present study aimed to test whether the amount of sugarcane straw kept on the soil surface under different soil and climatic conditions affects the decomposition and release of P and K, and how the position of the straw layer inside the mulch affects the P and K release dynamics. To achieve this objective, sugarcane straw from two field experiments was used. Experiment I was conducted on 2015/16, at five locations in the Rio Grande do Sul for 330 days and experiment II was conducted on 2017/18, at the UFSM Soil Department for 360 days. In both experiments, treatments consisted of 4, 8, and 12 Mg ha-¹ of straw, in a randomized complete block design with four replications. In experiment II, treatments with 8 and 12 Mg ha-¹ straw were divided into two (upper and lower) and three (upper, central, and lower) layers, respectively, with 4 Mg ha-¹ straw in each layer. In experiment I, seven collections were carried out and in experiment II, six straw collections were carried out, where the remaining dry matter (DM), P, and K were evaluated. The amount of straw did not affect decomposition, with 36.7 % of DM for the three amounts at 330 days (P > 0.05), and had a transient effect on the release of P and K in the first 150 days of evaluation. At 330 days, the amount of P and K released was 3.75 and 0.27 kg ha-¹ of K and P, respectively, for each ton of straw kept on the soil surface. However, the location affected the decomposition and release of P and K from the straw throughout the evaluated period, with the greater release of these nutrients in places with greater precipitation and average air temperature. The lack of effect of the amount of straw on the release of P and K can be attributed to the formation of a vertical gradient of nutrient release within the mulch, which can be observed through the results obtained in Experiment II, where mulch layers in contact with the soil present greater release of P, while layers exposed to the atmosphere tend to release K more quickly. The results of this study indicate that, regardless of location, partial straw removal does not affect straw decomposition and P and K release. However, partial removal reduces the amount of P and K recycled in the soil, a condition that requires attention to the balance of nutrients in sugarcane crops.