Objectives Excessive application of chemical fertilizers or unreasonable combined application of chemical and organic fertilizers is a common practice in greenhouse vegetable production. Therefore, a fixed-site greenhouse vegetable fertilization experiment was carried out to study effects of partial substitution of chemical fertilizer with organic amendments on nutrient and soil microbial biomass carbon (MBC) and nitrogen (MBN) distribution of soil aggregates in order to provide a scientific basis for high quality and efficient vegetable production in greenhouse and fertilizer reduction.
Methods The greenhouse field experiment was started in 2009, with 25% or 50% nitrogen of inorganic fertilizer replaced by nitrogen of corn straw or pig manure. Soil samples were collected from each plot at 0–20 cm depth during the uprooting stage of the 11th growing season (winter-spring cucumber) in the sixth year. The distribution and stability of soil aggregates, nutrient and MBC and MBN contents of soil aggregates were determined.
Results 1) The predominant size fractions in greenhouse vegetable soil were 250–1000 μm fractions and > 2000 μm fractions, which accounted for 32.0% and 38.4% on average by weight, respectively. Compared with 4/4CN treatment, organic amendments increased the proportion of >250 μm fractions. And soils amended with straw had a relatively large impact on the distribution of soil aggregates, following with a significantly improvement in soil aggregate stability. The MWD and GMD values in straw-treated soil were 6.1% and 11.2% higher than those in 4/4CN-treated soil, respectively. 2) The organic carbon content in organic-amended soil (3/4CN + 1/4MN, 2/4CN + 2/4MN, 2/4CN + 1/4MN + 1/4SN, 2/4CN + 2/4SN) was much higher than that in the 4/4CN treatment with the increasing ranges of 36.8%–89.6%, 34.9%–100.3%, 29.5%–69.2% and 21.7%–72.1% in < 250 μm fractions, 250–1000 μm fractions, 1000–2000 μm fractions and > 2000 μm fractions, respectively, which was respectively averagely increased by 69.8%, 76.6%, 56.9% and 49.2% compared with that of 4/4CN treatment. Actually, effects of the organic amendments on soil organic carbon, total nitrogen, nitrate nitrogen and available phosphorus were basically the same. 3) The organic carbon and total nitrogen in 250–1000 μm fractions and > 2000 μm fractions were the main sources of the organic carbon and nitrogen in the soil, which accounted for 34.1% and 35.2% of the total organic carbon stocks, and 34.0% and 36.4% of the total nitrogen stocks, respectively. 4) The content of soil nitrate nitrogen was higher in 250–1000 μm fractions and 1000–2000 μm fractions than that in other aggregate fractions. Soil readily available potassium, MBC and MBN contents increased with the increasing of soil aggregate-size, whereas soil available phosphorus decreased with the increasing of soil aggregate-size.
Conclusions The predominant size fractions in greenhouse vegetable soil were 250–1000 μm fractions and > 2000 μm fractions. And soil aggregate stability was significantly improved by partial substitution of chemical fertilizer with straw. Partial substitution of chemical fertilizer with organic amendments increased the content of organic carbon, total nitrogen, nitrate nitrogen and available phosphorus in soil aggregates. The organic carbon and total nitrogen were mainly stored in 250–1000 μm fractions and > 2000 μm fractions, while MBC and MBN contents increased with the decreasing of soil aggregate-size.
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