Thirteen Year Long Term Fertilization Effect on Soil P Balance and Sustainability of a Double Crop of Rice Grown in an Acidic Inceptisol under Sub-Tropical Climatic Situation

The goal of the study was to determine how long-term fertilizer management of a rice-rice production system affected the buildup or depletion of surface soil phosphorus (P) in a subtropical climate. The results showed that using FYM @ 5t ha-1 season-1 in conjunction with optimal NPK doses (80-40-60 kg ha-1season-1) resulted in significantly higher grain yield than using fertilizer alone at optimal, super optimal, or sub-optimal doses or optimal level along with lime/Zn/Zn+B/Zn+S. The use of NPK and FYM together resulted in the highest yield sustainability. Phosphorus balance measured by indirect method (apparent P- balance) and direct method (true P-balance) was positive in all the fertilized treatments and negative in control and 100% N treatment that received no P. High yielding treatments (NPK + FYM or NPK + FYM + Lime) maintained a relatively lower P balance than optimal or super optimal doses because of more removal through higher biomass production. The addition of 50% more phosphorus in 150% NPK treatment caused 9.77% more P accumulation than 100% NPK treatment. Between the two methods, the direct method registered more depletion in the two minus P treatments and less accumulation in all P-treated soils than the indirect method. In soil with a super optimal dose (150% NPK), there is the highest positive balance (25.18 kg ha-1 year-1) as compared to 16.5 kg ha-1year-1in the true method. Seasonal wet and dry grain yield and P uptake poorly correlated with apparent P balance, true P balance and total stock of P. Whereas, sustainability of dry season, wet season and system yield strongly correlated with apparent P balance (r=0.587*, 0.690** and 0.604*), true P balance (r=0.681**, 0.781** and 0.693**) and total stock of P (r=0.680**, 0.780** and 0.692**). Thus true P balance and the total stock of P in surface soil are better indicators of yield sustainability of each season and system as a whole; whereas, available P is closely related to the amount of residual fertilizer P in the soil. Consistent availability of the nutrient depends upon its total stock which in turn depends upon its balance. Knowledge of total stock and nutrient balance is very important for the sustainability of a particular cropping system and evaluating various nutrient management practices for their suitability. Further study on the composition of the total stock of P with respect to various P fractions and their relative contribution to P uptake will be useful for identifying the suitable management practice for P nutrition.