Effects of biochar addition on soil nitrogen retention and vegetable uptake in intensive production systems, China.

Abstract

China has a limited area of cultivated land per capita and an increasing population. Maintaining a high crop yield is essential to meet the large food demand and to assure grain self-sufficiency. With the pace of economic development, the demand for vegetables keeps growing. In Southern China, many of the fields used to grow vegetables were previously under paddy production. Compared to the paddy production system, the vegetable production system is intensive with excessive use of nitrogen fertilizer. Excessive nitrogen fertilizer application has changed soil chemical properties and nutrient dynamics, and thus created a negative impact on sustainable agricultural development. A preliminary study was conducted in the absence of nitrogen fertilizer to determine the effect of field utilization conversion on soil nitrogen uptake by pakchoi. It was found that soil pH values and organic matter content decreased with intensive vegetable planting and nitrogen leaching loss was higher from vegetable soils compared to that from paddy soils. Although the soil mineral nitrogen content in vegetable soils was higher than that in paddy soils, nitrogen uptake by plants from vegetable soils was lower than that from paddy soils, and decreased quickly in the later growing seasons. The lower plant nitrogen uptake was attributed to the high nitrogen leaching loss and soil acidity caused by the excessive application of nitrogen fertilizer in vegetable production systems. Therefore, it is imperative to find suitable approaches to mitigate nitrogen leaching loss and soil acidity in vegetable production systems and promote nitrogen retention and vegetable nitrogen uptake for sustainable productivity. Biochar is a fine-grained and porous substance produced through pyrolysis processes, under oxygen-free conditions, from a wide range of biomass. In recent years, biochar has received more attention with regard to its capacity to increase crop yields by ameliorating the soil environment and regulating nutrient processes. According to previous studies, biochar is an option for mitigating soil acidity and nitrogen leaching problems in vegetable soils due to its alkalinity and adsorption properties. However, studies of biochar addition to vegetable production systems have not been well documented. The effect of biochar addition on leachate volume is still lacking. Whether the mineral nitrogen retained by biochar can be re-used by plants is still unknown. Few studies have investigated the effect of biochar addition on nitrogen processes and soil acidity under continuous growing conditions. Therefore, with the aim of determining the effect of biochar on soil nitrogen retention and vegetable nitrogen uptake, pakchoi was planted in a pot experiment during four continuous growing seasons with three biochar addition rates (0, 1% w/w and 5% w/w). In the 1st, 2nd and 3rd seasons, pakchoi was applied with 15N-labelled urea and in the 4th season no nitrogen fertilizer was provided. The results of this study were presented in four parts (soil nitrogen retention, soil acidity, vegetable nitrogen uptake and a distinction between two nitrogen sources in vegetable nitrogen uptake i.e. nitrogen left in the soil and nitrogen loss). The main conclusions are as follows: Biochar addition significantly increased the soil mineral nitrogen content by enhancing nitrogen retention in soils and soil nitrogen mineralization. Part of the mineral nitrogen retained by biochar was still bioavailable for plant uptake in the soil. Biochar significantly reduced nitrogen leaching loss by decreasing leachate volumes and nitrate concentrations in the leachate. Biochar addition significantly ameliorated or retarded soil acidity by promoting soil pH buffering capacity, reducing soil acidification rates and maintaining soil bases contents induced by biochar. The mitigation of soil acidity was not only as a result of biochar’s natural alkalinity but can also be attributed to the altered nitrogen processes (promotion of plant nitrate uptake, reduction of nitrification and nitrate leaching and maintaining soil bases contents) with the addition of biochar. Biochar’s mitigation of soil acidity was partly dependent on its effect on soil properties (such as bases contents) and processes (such as nitrification, nitrate leaching and plant nitrate uptake) rather than its natural alkalinity. Biochar maintained pakchoi yields and nitrogen uptake during four growing seasons. The fertilizer nitrogen recovery efficency was improved with an increase in the recovery of fertilizer nitrogen in the soil and the decrease in the recovery of fertilizer nitrogen in leachate. Fertilizer nitrogen was the major source for pakchoi nitrogen uptake, soil residual nitrogen and nitrogen leaching loss, while nitrogen from soil mineralization was the major nitrogen source for biochar retention. When nitrogen fertilizer was absent in the 4th season, the nitrogen fertilizer left in the soil from the 1st to 3rd seasons decreased sharply and fertilizer nitrogen retained by biochar was simultaneously released. The conclusion was that biochar addition could promote soil nitrogen retention and maintain high nitrogen uptake by vegetables in continuous growing seasons. However, the comprehensive effect of biochar on nitrogen loss still needs to be assessed before recommending extended utilization of biochar in vegetable production systems in China.