Effects of continuous application of non-hazardous sewage sludge compost on fertility of sandy fluvo-aquic soil

Objectives  In order to provide theoretical basis and technical support for the utilization of harmless sludge, a study was conducted to investigate the effects of non-hazardous sewage sludge compost on soil fertility and environment quality.

Methods  Experiments were conducted in wheat-maize rotation system during 2013 to 2015. At the base of applying N 225 kg/hm², P₂O₅ 86 kg/hm² and K₂O 113 kg/hm² (CK), sewage sludge compost was applied at rates of 15 t/hm² (W1), 30 t/hm² (W2) and 45 t/hm² (W3) before the sowing of wheat and maize. After every harvest, the soil samples of 0–20 cm depth were collected, basic physical and chemical properties, heavy metal contents and soil microbial biomass carbon and nitrogen contents (SMBC and SMBN) were analyzed. The individual soil fertility index of pH, organic matter, total N, available P and available K (IFI_i) were calculated, and the soil integrated fertility index (IFI) was obtained using modified Nemero formula to evaluate the soil fertility quality.

Results  1) IFI in sludge-treated soils increased with the experimental years. Compared with CK, IFI of W1, W2 and W3 treatments were significantly (P < 0.05) increased by 57.3%, 95.2% and 127.5% in 2015 maize season, respectively, indicating that applying sewage sludge compost effectively improved the soil fertility. 2) SMBC and SMBN contents were increased with the experimental years as well. The SMBC and SMBN contents of the same treatment in the same harvest season were significantly improved from 2013 to 2015. In 2015, the highest SMBC content was in wheat season, which were significantly increased by 109.9%, 176.2% and 216.8% in W1, W2 and W3, compared with the CK; the highest SMBN content was in maize season, which were significantly raised by 55.6%, 100.5% and 162.3% in W1, W2 and W3 treatments, compared with the CK (P < 0.05). 3) The heavy metal contents in both sludge-treated soils and plant grains were far below the national environmental quality standards GB15618–1995 and the limits in the “agricultural product quality safety regulations of the state”. Referring to the standard of the Soil Fertility Classification Standard (NY/T 391–2000), soil fertility of W2 and W3 treatments met the first grade after three year’s continuous application.

Conclusions  Applying high rate of non-hazardous sewage sludge compost is proved effective in improvement of soil fertility and soil microbial properties in a sandy fluvo-aquic soil. Under the experimental condition, applying 45 t/hm² of sewage sludge compost for three years is the most prominent treatment which increases fluvo-aquic soil fertility to grade I, without leading to obvious heavy metal accumulation in soils and grains. Long term monitoring is still needed for the food and ecosystem safety in the future.