Phosphorus Biogeochemical Cycling in a Sugar Cane Agroecosystem

The annual harvest of sugarcane plantations together with the burning of the crop before harvest, a common practice of management of sugarcane plantations in South America, leads to the loss of significant amounts of nutrients in those agroecosystems. Thus prescribed burning operations could progressively diminish the level of soil organic matter and increase nutrient deficiency in soils of sugar cane agrosystems. This study is an attempt to quantify the P distribution during the period of growth in a plantation of sugar cane (Saccharum officinarum) located near San Felipe, Yaracuy State, Central, Venezuela focusing on processes related to the cycling of the element as affected by burning operations. The work was performed in 4.5 ha experimental plots planted with the varieties Puerto Rico (PR) 1028 and Venezuela (V) 58-4. The principal flows of phosphorus, as well the quantities of this element in the soil-plant components were measured throughout the growing cycle of the crop (third ratoon). The inputs through precipitation (wet and dry) were high, that was associated with the intense agricultural (prescribed burning) and industrial activities occurring in the area. The annual balance for both varieties was negative (-17.31 and -23.63 kg ha^-1 for V 58-4 and PR 1028, respectively). The negative budget is mainly due to the important amounts of P that are exported with the cane stems. The losses must be compensated through fertilization; nonetheless, preliminary results indicated no response to P dressing, suggesting that in the studied mollisols the internal processes e.g., Organic-P (Po) mineralization and P solubilization efficiently operate generating important available P levels. It was also found that the burning of the sugar cane plantation plays an important role in the recycling of phosphorus, since 25-28 % of the P requirements of the varieties are reincorporated into the soil from the ashes coming as bulk deposition.