No abstract

We analyzed the trends in phosphate (P) demand from 1966 to 1999, estimated the P flow in farmland in 1997, and evaluated the risk of environmental damage in Japan. Chemical P fertilizer demand per unit farmland area increased from 106 kg ha-1 in 1966 to 152 kg ha-1 at the end of the 1970s, then decreased 143 kg ha-1 after the mid 1980s to 117 kg ha-1 in 1999. This trend reflected recommendations on the use of large inputs to increase soil fertility until the 1970s, and the reduction of production costs since the 1990s. In 1997, the amount of P applied to farmland soils in Japan was 6.3 times higher than crop uptake. Outflow from farmland was estimated to account for only 0.6% of the applied P. Most of the applied P remained in farmland soil. P in farmland soils increased the risk of eutrophication by flooding and surface run off. Seventy six percent of applied P was in the form of chemical fertilizer. Chemical fertilizer was applied at a higher rate to fruit and nut fields, vegetable fields, and industrial crop fields than to paddy fields. The remaining P resulted in a high availability P in fruit and nut fields and horticultural fields. The amount of P in livestock excreta corresponded to half of the amount of chemical P fertilizer applied. One-third of the livestock excreta was not utilized in 1997, which was the main source of environmental load associated with agriculture. Therefore, promotion and alternative use of livestock excreta are important to reduce the environmental load, while uneven regional distribution of livestock excreta makes it more to achieve this objective difficult. Maintenance of optimal soil fertility by using chemical P fertilizer and manure is important for saving limited P resources and for securing a low environmental risk.

A reduction in chemical phosphate (P) fertilizer application to farmland from 137.6 kg P ha -1 in 1985 to 99.0 kg P ha -1 in 2005 and in manure application from 42.4 kg P ha -1 in 1985 to 32.8 kg P ha -1 in 2005 did not reduce crop P uptake, which averaged 27 kg P ha -1 over the period. Phosphate balance on farmland declined from 153.0 kg P ha -1 in 1985 to 105.4 kg P ha -1 in 2005 while livestock excreta disposal increased from 12.7 kg P ha -1 in 1985 to 23.7 kg P ha -1 in 2005. As a result, residual P associated with agriculture declined from 165.8 kg P ha -1 in 1985 to 129.1 kg P ha -1 in 2005. Phosphate utilization efficiency increased from 15.7% in 1985 to 20.1% in 2005. Median, minimum and maximum values of P flows by region showed similar trends. Phosphate input and withdrawal through crop production by region were not related to regional nitrogen (N) input and withdrawal through crop production. Although non-utilized P associated with agriculture has declined nationally and regionally, it is still higher than that in foreign countries, because of high chemical P fertilizer inputs and low crop yield withdrawal. Because soil P fertility was often sufficiently high previous large P surpluses, reducing P applications did not affect crop yields. Crop P uptake was less than half that of crop N yield. These results indicate that P inputs, especially by chemical fertilizer, for crop production could be reduced, thereby reducing negative environmental effects such as eutrophication of soil and water and conserving limited P resources.

We calculated the residual nitrogen (N) from agricultural production on national and regional scales in Japan for 1985, 1990, 1995, 2000, and 2005 and tested the suitability of the N concentrations applied in agricultural production systems using residual N and excess water as indicators of negative effects on groundwater. Chemical fertilizer application declined consistently during the period from 1985 to 2005, while the application of livestock manure peaked in 1990 and declined thereafter. Crop production remained relatively constant, and surplus N on farmland (total input minus total output of N) declined during this period. Although the disposal of excreta N increased, the residual N from agricultural production (surplus N plus disposal of excreta N) declined consistently during this period. However, this trend was not consistent at the regional level because residual N was largely affected by the movement of chemical fertilizer and livestock excreta, which varied with each farm and region. A comparison of residual N per farmland area in 1985 and 2005 indicated that regions were becoming bipolarized, i.e., certain regions exhibited reduced residual N while others exhibited the opposite trend, primarily due to an intensification of livestock production. A significant correlation was observed between residual N in excess water (precipitation minus potential evapotranspiration within the regions), and the percentage of observation wells exceeded[10 ppm of N from 2000 to 2005. Although this correlation had a large dispersion, residual N levels in excess water indicated the risk of water contamination by N.

Trends in nitrogen (N) and phosphate (P) balance for several crops were calculated for the nation and by prefecture for 5-year periods from 1985 to 2005. Prefectural chemical N and P fertilizer applications for paddy rice and upland crops declined but applications for vegetable crops increased during the period like as national trends. Prefectural chemical N and P applications for tea, orchard and forage remained unchanged in line with national trends. Manure N and P applications for each crop did not follow the trends for chemical fertilizer. Although chemical fertilizer application declined, N and P crop withdrawal for paddy rice, upland crops and tea increased as a result of optimizing fertilizer timing and placement. Nitrogen and P balance for each crop indicated a surplus; P surplus was larger than N surplus, because of higher P input and lower crop P withdrawal. Chemical N fertilizer determined N surplus except for forage, which was determined by manure application. Therefore N surplus on paddy rice, upland crops, orchard and tea declined and increased on vegetables. Forage recorded an N deficiency in two 5-year periods nationally because of low manure input. Because P balance was also determined by chemical P fertilizer, the P surplus for paddy rice and upland crops declined and the P surplus for vegetables increased during the period. Total P surplus was reduced on paddy rice and upland crops. Trends for chemical fertilizer, manure and N and P balance varied widely among prefectures, especially for P. Crop withdrawal of N and P varied relatively little. Although prefectural trends did not always follow the national trend, we could conclude that the negative environmental effects of Japanese crop production have diminished recently, and an improvement in N and P balance in vegetable production can be expected.