ObjectivesThe study was designed to explore the influences of long-term fertilization on soil pH, Eh, and organic matter based on the long-term fertilization experiment in Shenyang Agricultural University. The study also aimed to investigate variations of free iron oxide, amorphous iron oxide, total ferrous iron, and available iron in soil under different fertilizer treatments, and to find the relationships between different iron fractions and pH, Eh and organic matter.
MethodsThe treatments selected in the experiment were CK (no fertilizer), N (nitrogen fertilizer), NP (nitrogen + phosphorus fertilizers), NPK (nitrogen + phosphorus + potassium fertilizers), M (organic fertilizer), MN (organic fertilizer + nitrogen fertilizer), MNP (organic fertilizer + nitrogen + phosphorus fertilizers) and MNPK (organic fertilizer + nitrogen + phosphorus + potassium fertilizers). Soil samples were collected from 0-20 cm top soil in different fertilization plots at the harvest of soybean in 2014. We determined soil pH, Eh, and organic matter, and the contents of free iron oxide, amorphous iron oxide, total ferrous iron, and available iron in these samples.
ResultsCompared to that in original soil in 1979, the present soil pH values for all treatments were significantly decreased by 0.6-1.4 units. The contents of soil organic matter were decreased by 11.5% in the no fertilizer treatment (CK), slightly decreased in the chemical fertilizer treatments, and significantly increased in the organic fertilizer treatments. The contents of available Fe for all treatments were markedly increased, and most notably for the chemical and organic fertilizers co-used treatments. Compared to that in the CK in 2014, the soil pH was the lowest in the N fertilizer treatment, but raised again when the P, K and/or organic fertilizers were co-used, and most notably when the organic fertilizer was used in soil. The soil Eh values were increased in the chemical fertilizer treatments, while the values were decreased in the organic fertilizer treatments. The contents of free iron oxide and available iron were increased and the contents of total ferrous iron were decreased in the chemical fertilizer treatments. The contents of free iron oxide were decreased and the contents of total ferrous iron and available iron were increased in the organic fertilizer treatments.
ConclusionThe long-term fertilization can cause a significant decrease of soil pH and significant increase of available iron. The long-term no fertilization can cause a significant decrease of soil organic matter content. The use of single N fertilizers will lead to vital decline of soil pH, but will not when combined with the phosphorus, potassium and/or organic fertilizers. The use of organic fertilizers will induce the decrease of soil Eh, benefit the transformation of oxide iron to ferrous iron, which is further helpful for the increase of soil available iron.
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