Abstract
Phosphorus in agro-ecosystems has attracted much attention due to its impact on the nutrient supply of plants and the risk of loss of non-point source pollution. This study investigated the fraction distribution and release of phosphorus from soil aggregates structure under different land uses (rice, maize and soybean). The soil aggregates were characterized as large macro-aggregates (L-mac, >1 mm), small macro-aggregates (S-mac, 0.25–1 mm), micro-aggregates (MIC, 0.053–0.25 mm) and silt clay (SC, <0.053 mm) with the wet-sieving method. A sequential chemical extraction scheme was used to separate phosphorus into labile inorganic phosphorus (L-Pi), labile organic phosphorus (L-Po), moderately labile organic phosphorus (Ml-Po), iron-aluminum bound phosphorus (Fe.Al-P), calcium-magnesium bound phosphorus (Ca.Mg-P), humic phosphorus (Hu-P) and residual phosphorus (Re-P). Experimental results indicated that soil aggregates were mainly S-mac and MIC, followed by L-mac and SC, and they accounted for 52.16%, 25.20%, 14.23% and 8.49% in rice fields, 44.21%, 34.61%, 12.88% and 8.30% in maize fields, and 28.87%, 47.63%, 3.52% and 19.99% in soybean fields, respectively. Total nitrogen (TN), soil organic matter (SOM), Fe and Mn in soil aggregate fractions decreased with the reduction in soil aggregate grain-sizes. For phosphorus fractions (P-fractions), Fe.Al-P and Re-P tended to condense in L-mac and S-mac. MIC and SC were the primary carriers of Ca.Mg-P. Adsorption isotherm simulation results demonstrated that L-mac and S-mac have a strong capacity to retain phosphorus. In rice fields, phosphorus bioavailability and utilization rate were high. However, the P-fractions there were easily changed under aerobic-anaerobic conditions. Therefore, the risk of phosphorus loss during drainage should be given considerable attention.
Highlights
Agricultural non-point source (NPS) pollution has seriously jeopardized the quality of the aquatic eco-environment because of the excessive amount of chemical fertilizers used in agricultural production [1,2]
On the base of the hypothesis that different cultivated crops affect the structure and stability of soil aggregates and affect the retention-release mechanism of P-fractions in there, we studied the difference in the mass proportion of soil aggregates with different grain-sizes and their chemical properties
labile organic phosphorus (L-Po) is positively correlated with Total nitrogen (TN) and soil organic matter (SOM) at p = 0.01, and it is positively correlated with Fe and Mn at p = 0.05
Summary
Agricultural non-point source (NPS) pollution has seriously jeopardized the quality of the aquatic eco-environment because of the excessive amount of chemical fertilizers used in agricultural production [1,2]. Data from the China Statistical Yearbook reveal that annual chemical fertilizer application in China is 2.6-fold as that of the United States and 2.5-fold as that of the European. Phosphoric fertilizer applied to farmland can form insoluble phosphates adsorbed by soil minerals or retained by organisms, thereby resulting in its quarterly utilization rate of only 10–20% [5]. 60% of China’s underground water is unsuitable for human consumption and China’s per capita renewable freshwater is only one-third of the world’s average level [1,4]. The proportion of water eutrophication caused by agricultural. Public Health 2019, 16, 212; doi:10.3390/ijerph16020212 www.mdpi.com/journal/ijerph
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