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Crop species and their varieties vary in phosphorus (P) requirements for optimum production and response to P application. As crop recovery of added P often ranges from 10 to 40%, the rest accumulates in soil and may create potential for P leaching, depending upon the soil characteristics, duration of P applications, and cropping systems. Accumulation and distribution of Olsen P (plant-available labile P), total inorganic P, and total organic P were investigated in soil profiles of three field experiments differing in rate (9-44 kg P ha-1), frequency (applied once or twice annually), and duration (4-34 years) of fertilizer P applications, crop rotations, soil characteristics, and irrigation pattern (upland irrigated and flooded-rice crop) in a subtropical region. Profile samples were collected from soil depths of 0-15, 15-30, 30-60, 60-90, 90-120, and 120-150 cm of different treatments in these experiments and analyzed for different forms of P and soil characteristics. The results revealed that (i) annual applications of fertilizer P either to one crop (alternative-applied P) or to both crops (cumulative) led to the accumulation of residual fertilizer P in the form of Olsen P, varying from 44 to 148 kg P ha-1, and the magnitude of accumulation was proportional to applied fertilizer P rate, frequency, and duration; (ii) majority of residual fertilizer P accumulated as inorganic P (74-89%) followed by organic P (11-26%) and Olsen P (9-19%), illustrating that the inorganic P pool is a major sink for fertilizer P; (iii) application of fertilizer nitrogen (N) and potassium (K) alone or in combination with fertilizer P did not affect residual fertilizer P accumulation in soil profile; (iv) incorporation of farmyard manure enhanced the P enrichment of soil profile; (v) irrigation pattern, soil pH (7.1-7.7), and calcium carbonate (CaCO3) (trace-0.33%) did not influence P movement to deeper soil layers; silt, clay, and soil organic C (SOC) showed strong relationships with Olsen P (r = 0.827, 0.938, and 0.464, P < 0.01) and enhanced the retention of labile P in the plow layer; and (vi) only 6-29% total residual P moved beyond 30 cm deep in fine-textured soils under 22-year rice (Oryza sativa L.)-wheat (Triticum aestivum L.) and 34-year maize (Zea maize L.)-wheat rotations, whereas 41, 27, 20, 9, and 3% were located in soil layers 0-30, 30-60, 60-90, 90-120, and 120-150 cm deep, respectively, in coarse-textured soil profile under 4-year peanut (Arachis hypogaea L.)-sunflower (Helianthus annuus L.) field. These findings confirmed that interplay between the fertilizer P management (alternative vis-a-vis cumulative P application and optimal vis-a-vis excessive rates of fertilizer P in different crop rotations), amount of labile P accumulated in soil profile, and soil characteristics (silt, clay, and SOC) largely controlled the downward movement and resultant potential for P leaching in subtropical irrigated soils. [ABSTRACT FROM AUTHOR]

Long-term continuous cereal cropping systems without returning crop residues to soil has led to depletion of inherent soil fertility in semiarid subtropical soils of South Asia. The inclusion of legumes and their residues assumes great significance to restore sustainability. Groundnut (Arachis hypogaea L.) and sunflower (Helianthus annus L.) are important oilseed crops of the region and have the feasibility to be grown in annual double crop rotation with the expansion of irrigation facilities. A 4-year field experiment with irrigated groundnut-sunflower rotation was conducted on a loamy sand soil (Typic Haplustept) to evaluate the effects of different rates of fertilizer N (0, 30 and 60 kg N ha-1) and P (0, 20, 30 and 40 kg P2O5 ha-1 corresponding to 0, 8.7, 13 and 17.5 kg P ha-1, respectively), applied either to one crop (direct for current crop and residual for succeeding crop) or to both crops (cumulative) each year, with or without 4 t ha-1 of groundnut crop residues (CR) on crop yields, nutrient uptake, fertilizer use efficiency and soil productivity parameters (pH, organic carbon, available N and P). The residual effects of P and CR were assessed on the succeeding groundnut crop. Winter-grown sunflower responded significantly and consistently to applied fertilizer N and P. Seed yield of sunflower increased significantly up to 60 kg N and 30 kg P2O5 ha-1 and there was no further increase with directly applied 40 kg P2O5 ha-1. Cumulative application of P produced sunflower yields comparable with those from direct applications. Incorporation of groundnut residue in conjunction with 30 kg N ha-1 significantly increased the sunflower yields that were statistically at par with those obtained with the lone application of 60 kg N ha-1. All through 4 years, the combined application of CR with 60 kg N ha-1 and 30 kg P2O5 ha-1 further improved the yield potential of sunflower by 10-11% and of succeeding groundnut by 10-24%, illustrating the benefit that fertilizer N and P could not achieve. Residual fertilizer P (30 kg P2O5 ha-1) produced optimum yields of groundnut, which were comparable with that from directly applied 20 kg P2O5 ha-1. After 4 years of study, soil organic carbon swelled by 22% and Olsen-P increased from initial 12 kg P ha-1 to 24-43 kg P ha-1 in fertilizer and CR-treated plots. These results illustrated that in groundnut-sunflower rotation, (1 )60 kg N and 30 kg P2O5 ha-1 are adequate for the sustaining optimal sunflower production; (2) whereas the application of fertilizer P to sunflower crop is required, P requirement of groundnut can be met from soil-derived and residual fertilizer P; and (3) incorporation of groundnut crop residues in conjunction with the adequate rates of N and P has complementary effects in maximizing the yields, uptake of N and P, and nutrient use-efficiency in sunflower as well as its residual effects on succeeding groundnut crop, and in sustaining long-term soil productivity. [ABSTRACT FROM AUTHOR]