Crop yields, soil fertility and phosphorus fractions in response to long-term fertilization under the rice monoculture system on a calcareous soil

A 14-year field trial was conducted on a calcareous soil to evaluate the effects of continuous rice (Oryza saliva L.) cropping and fertilization on crop yield, soil fertility and phosphorus fractions. The application of N and P enhanced rice yields, while K had no yield-increasing effect because of large available soil K resource. The soil organic carbon remained at a stable level except for a decrease in the unfertilized treatment after 1988. Total N did not show clear changes with time or between treatments despite some fluctuation. The available soil K trend after 1985 as well as the comparison between treatments indicated clearly decreasing available soil K, particularly in the NP treatment. Available soil P significantly decreased in the P-omitted treatments, but remained at a stable level in the P-applied treatments with time. Of the total inorganic P (Pi), Ca phosphates (Ca-P) were the dominant Pi forms, accounting for 69-71%, followed by Fe phosphates (Fe-P), P occluded within Fe oxides (O-P) and then Al phosphates (Al-P). Of the Ca-P fractions, Ca-2-P [CaHPO(4)(.)nH(2)O], Ca-8-P [Ca8H2(PO4)(6)(.)nH(2)O] and Ca-10-P [Ca-10(PO4)(6)(.)(OH)(2)] accounted for 3.1-6.2, 5.8-6.4 and 87-91%, respectively. The NK treatment had significant Ca-2-P depletion with time compared with the NPK treatment, and the similar trend was observed for Cas-P despite decreasing Ca-8-P with time for the two treatments. In the NK treatment, Ca-2-P and Ca-8-P had a good correlation with Olsen-P. The significant changes with time were not observed for Fe-P, O-P and Ca-10-P. The P application caused a weak accumulation of Fe-P, O-P and Ca-10-P, but had no significant effect on total P-i over time. The results suggest that (i) P application is indispensable to maintain high yields of rice under N application and (ii) there was a substantial P release from Ca-2-P and Ca-8-P linked to P uptake by crops. Hence, soil indigenous P supply, P transformation and the yield responses to fertilizer P application must be synthetically considered to optimize fertilization strategies for irrigated rice production on the calcareous soil at the experimental site. (C) 2003 Elsevier B.V. All rights reserved.