Field site description
The medium-term experiment was located at the Xinzhu country (29 °C 01༇10༂N, 119 °C 28༇10༂E and 80 m above mean sea level), Jinhua city, Zhejiang province, China. The prevailing climate of the experimental is a subtropical monsoon climate with four distinct seasons. Mean annual temperature is 17.5 °C, and mean annual rainfall is 1424 mm. The soil at the experimental site belongs to yellow clayey paddy soil. Mean aggregate size distribution (per 100 g) was 37 g sand, 35 g silt, and 28 g clay.
Experimental layout and sampling
The experiment was initiated in 2011 and included three treatments with three replicates. Each plot size was 5 m width and 6 m length. Treatments were: (1) inorganic N-P-K fertilizers application alone (NPK); (2) the NPK fertilizers plus rice straw at 3 Mg ha− 1 per rice-growing season (NPK + RS); (3) controlled-release blended fertilizer plus cattle manure at 4.7 Mg ha− 1 per rice-growing season (CRF + CM). Before imposing experimental treatments, the soil contained 28.1 g kg− 1 soil organic matter, 1.75 g kg− 1 total N, 117 mg kg− 1 alkali-hydrolyzable N, 11.6 mg kg− 1 available P, 78 mg kg− 1 available K, and pH was 5.24.
Common urea, superphosphate, and potassium chloride were the sources of N, P and K, respectively. Controlled-release blended fertilizer (CRF, N-P-K, 24-12-12) was provided by Kingenta Ecological Engineering Group Co., Ltd., Shandong, China. All three treatments received the same rates of inorganic nutrients (180 kg N, 90 kg P2O5, and 120 kg K2O per hectare) in each rice-growing season. For the NPK alone and NPK + RS treatments, the complete doses of P and K chemical fertilizers were applied as a basal dressing. Nitrogen fertilizer was applied as split applications at ratios of 40%: 30%: 30% (basal: tillering: booting) for the early rice, and at ratio of 40%: 60% (basal: tillering) for the late rice. For the CRF + CM treatment, CRF was applied as a single basal application. Rice straw and cattle manure were applied on the soil surface once before rice transplanting. On average, rice straw contained 44% C, 0.9% N, 0.1% P, and 2% K on a dry weight basis. Cattle manure contained 29% C, 2% N, 1% P, and 1.2% K on a dry weight basis.
Tillage operations for each plot include deep plough at the depth of 22–25 cm in April and shallow plough at the depth of 10–15 cm in late–July before transplanting every year. Early rice was transplanted in late-April and harvested in late-July. Late rice was transplanted at the end of July and harvest in early-November. The fields remained fallow between harvest and transplanting (from November to next year April). Seedlings were transplanted at four plants per hill in a planting pattern of 16.5 cm × 19.8 cm for early rice, and two plants per hill in a planting pattern of 19.8 cm × 19.8 cm for late rice each year. Early rice variety was “Jinzao 09” and late rice variety was “Yueyou 9113”.
Soil samples from each plot were taken at three stages: tillering stage (May 19th, 2018), jointing stage (June 10th, 2018), and after rice harvest (July 25th, 2018). Specific soil sampling methods are as follows: Ten whole plants with their roots were randomly selected and extracted from soil in each plot. Then, after gentle shaking off the loosely adherent soil, the tightly adherent soil was carefully collected regarded here as rhizosphere soil. The unvegetated soil (0–20 cm) adjacent to the rice plants was sampled as bulk soil (Ai et al. 2013). Ten soil cores (diameter 5 cm) from each plot were mixed to form a composite soil sample. Thus, total 54 composite soil samples were collected (3 treatments × 2 soil fractions × 3 replicates × 3 rice growth stages). The samples were placed in hard plastic boxes (180 mm × 120 mm ×70 mm) and immediately transported to the laboratory. After removing residual roots, soil samples were passed through 8.0 mm sieve and stored at 4 °C before analysis.
Soil aggregate fractionation and chemical analysis
Soil samples were physically fractionated into different classes of aggregates by wet sieving (Kemper et al. 1986) following the method described by Wang et al. (2015). In wet sieving, 100 g of < 8 mm soil sample was immersed in water on top of a stack of sieves (5–, 2– and 0.25 mm). The stack was moved up and down by 3 cm, 30 times per minute for 30 min. Soil aggregate fractions remaining on the sieves were oven-dried at 50 °C, weighed and stored at room temperature for C and N analyses. The ratio of aggregate mass on each sieve to the total soil mass was calculated.
The mean weight diameter (MWD) was calculated by the following equations (Pinheiro et al. 2004):
where Xi is mean diameter of each size fraction and Wi is the weight of aggregate in that size range as a fraction of the total analyzed sample weight, and n is the number of the sieves.
Total soil organic carbon (TOC) and total N concentrations were analyzed by dry combustion method using a Vario MACRO C/N elemental analyzer (Elementar Corp, Germany) (Tian et al. 2012).