Low risks of toxicity from boron fertiliser in oilseed rape–rice rotations in southeast China

Abstract

Yields in intensive rice (Oryza sativa L.) -based rotations in Asia are stagnating or declining because of decreasing nutrient availability and depletion of soil nutrient reserves. In the rape (Brassica napus L.)–rice–rice cropping rotation of southeast China, where boron (B) deficiency is widespread and B fertiliser is needed to correct it, our objective was to evaluate the risks of fertiliser-induced B toxicity in oilseed rape and in rice. Response of oilseed rape to B fertiliser application at rates up to 6.6 kg B ha-1 was studied in seven field experiments on three contrasting soils of Zhejiang province, alluvial, red and blue-purple soils. The effects of up to 3.3 kg B ha-1 in 1 year, 6.6 kg B ha-1 in 2 years or 9.9 kg B ha-1 in 3 years were studied on oilseed rape and the one or two rice crops grown immediately after rape in each annual crop rotation. Soils varied in initial hot CaCl2-extractable B in the 0–15-cm layer from 0.24 to 0.99 mg kg-1. At the stem elongation stage of oilseed rape, 3.3 kg B ha-1 depressed shoot dry matter on soils with low clay and organic matter content. However, the subsequent effects of high fertiliser B on seed yield were minimal even at 6.6 kg B ha-1. Moreover, the application of a total of 6.6 or 9.9 kg B ha-1 as successive annual applications of 3.3 kg B ha-1 to oilseed rape generally had no negative effect on seed yield of oilseed rape. The single exception was on a sandy alluvial soil where a total of 6.6 kg B ha-1 in 2 years slightly depressed seed yield of oilseed rape. Grain yields of rice crops grown in rotation after oilseed rape were unaffected by B applications up to 6.6 kg ha-1. The minimal effects of a total of 6.6 kg B ha-1 applied over 2 years on seed yield were consistent with the modest increase in hot CaCl2-extractable B levels. It is concluded that there is limited risk of B toxicity from the use of borax fertiliser at up to 4–8 times recommended rates in rape–rice cropping rotations in southeast China. The low risk of B toxicity can be attributed to the relatively high B removal in harvested seed, grain and stubble, the redistribution of fertiliser B by leaching in the 0–60 cm layer and to B sorption.