Effects of manures on carbon, nitrogen, phosphorus content and bacterial community in reclaimed soil aggregates

Objectives Applying manures can effectively improve the fertility, aggregates number and microbial activities of reclaimed soils. We investigated the effects of different manures on the carbon, nitrogen and phosphorus contents and bacteria communities in the aggregates of a reclaimed soil.

Methods A field experiment was carried out in the coal mining subsidence area of Xiaoyi City, Shanxi Province for 5 consecutive years. The experiment treatments contained blank control, chicken manure, pig manure, cow manure, and chemical fertilizer (CF). The total annual nutrient inputs were N 200 kg/hm², P₂O₅ 100 kg/hm², and K₂O 100 kg/hm² for all the four fertilizer treatments, the manure application amount was calculated by P₂O₅ 100 kg/hm², the gaps for N and K₂O were supplemented with chemical fertilizers. After maize harvest on the fifth year, 0−20 cm soil samples were collected, and divided into three particle sizes (>2 mm, 0.25−2 mm and <0.25 mm) using dry screening method. The soil organic carbon (SOC), dissolved organic carbon (DOC), total nitrogen (TN), total phosphorous (TP), available phosphorous (AP), microbial biomass carbon, nitrogen and phosphorous (SMBC, SMBN and SMBP) in each particle size were analysed using conventional methods, and the relative abundance of bacterial communities were calculated using 16S rRNA high throughput sequencing method.

Results 1) Compared to CK, CF significantly reduced, while manures increased the proportion of >2 mm aggregates, and the increment in chicken manure treatment reached significant level. In addition, manures significantly increased the proportion of >2 mm aggregates, compared to CF, the increment in chicken, pig, and cow manure treatments were 35.56%, 22.32% and 20.25%, respectively. Fertilization substantially elevated the C, N, and P contents in all the three particle sizes of aggregates, manures exhibited more pronounced increment than CF, with increase of SOC 50%, TN 15%, TP 43% on average, respectively. In >2 mm aggregates, manures resulted in 42% higher DOC than CF, and pig manure resulted in 28% and 40% higher DOC than chicken and cow manures; in 0.25−2 mm aggregates, manure treatments had 41%, 73%, 42% and 260% higher TN, TP, SMBN, and SMBP than CF, respectively, and the increment of above indexes were in order of chicken manure>pig manure≥cow manure. In <0.25 mm aggregates, the increments of manures over CF were by 75% in organic C, 373% in AP and 124% in SMBC, respectively. Among the manures, the improvement of TN, TP, SMBN and SMBP by fertilization all showed the order of chicken manure>pig manure≥cow manure≥chemical fertilizer, while the improvement of SOC, DOC and SMBC contents were the most significant in pig manure. 2) Actinobacteria, Proteobacteria, Chloroflexi, Acidobacteria and Bacteroidetes were the dominant phyla in soil. Fertilization significantly increased the bacterial diversity and richness in 0.25−2 mm aggregates. Compared to CK, fertilization reduced the relative abundance of Acidobacteria by 17.8%−110%, increased that of Proteobacteria by 8.98%−43.5%, and Bacteroidetes by 19.9%−56.0%, and the increment by manures reached significant levels. PICRUSt function prediction showed that manures significantly increased the abundance of functional genes related to C, N and P metabolism, with the greatest elevation in chicken manure treatment.

Conclusions Manures application could improve the stability of reclaimed soil structure, and increase carbon, nitrogen and phosphorus contents in aggregates. The effects are varied among manures, pig manure is the most effective in carbon content, and chicken manure is in phosphorus content, bacterial diversity and abundance.