Annual variation of active carbon and nitrogen contents and the related factors in red paddy soils of subtropical China

Objectives Active carbon and nitrogen play important roles in soil carbon and nitrogen cycle. This study aimed to determine the difference of active carbon and nitrogen contents in soils affected by fertilization and other factors through the year in the subtropical double-rice region of China.

Methods A long-term field experiment on reddish paddy soil was conducted. The three treatments were CK (non-fertilizer), NPK (N, P, and K fertilizer application), and NPKM (NPK fertilizer, milk vetch and rice straw application). The characteristics and variation trends of inorganic nitrogen (NH₄⁺-N and NO₃^–-N), dissoluble organic carbon and nitrogen (DOC and DON) and microbial biomass carbon and nitrogen (MBC and MBN) in soils were investigated during the rice growing and fallow periods.

Results The NH₄⁺-N contents in paddy soils fluctuated in all treatments and showed three general trends: decreased during the early rice stage, stable during late rice period, and increased then decreased during the fallow period. NH₄⁺-N, the main form of inorganic nitrogen in soils, was in range of 14.9–31.6 mg/kg, and no significant difference was found between the rice growing period and fallow period. The contents of soil NO₃^–-N, less than 3.0 mg/kg, were far lower than those of NH₄⁺-N contents, and there was a reciprocal relationship between them (P < 0.01). Both of soil DOC and DON had a similar annual variation with a declining trend in the rice growing period and an increasing first and decreasing later trend in the fallow period. The contents of MBC and MBN in soils were 463–701 mg/kg and 31.1–52.4 mg/kg during the non-rice growing period, which were increased by 25.4%–36.9% and 62.8%–125.9%, compared to those in the rice-growing period, respectively. Overall, the total contents of active carbon (DOC + MBC) and nitrogen (NH₄⁺ -N+ NO₃^– -N+ DON + MBN) in soils in the non-rice growing period were increased by 10.8%–19.6% and 10.3%–34.8%, respectively, compared with those in the rice-growing period. The total contents of active carbon or nitrogen in soils in the NPKM treatment were significantly higher than those in the NPK and CK treatments(P < 0.05). The active carbon and nitrogen contents in the NPKM treatment were increased by 53.8% and 81.2% in the growth season, and by 43.5% and 63.2% in the fallow season, compared with those in the CK treatment. No significant differences were found between the NPK and CK treatments during the two periods. During the fallow period, soil NH₄⁺-N, DOC, and DON had significant and positive correlations with SOC and TN; Soil NH₄⁺-N and soil NO₃^–-N were significantly correlated with soil temperature at 5 cm (P < 0.01 and P < 0.05); Soil DOC and DON were significantly correlated with soil water contents (P < 0.01 and P < 0.05).

Conclusions Combined application of milk vetch and rice straw and chemical fertilizers could significantly increase the contents of active carbon and nitrogen in soils, which were positively correlated with soil organic carbon and total nitrogen contents. Compared with the rice growing period, the contents of active carbon and nitrogen in soils maintained high levels during the fallow period, which were affected by SOC and TN, and environmental factors, i.e., soil NH₄⁺-N and soil NO₃^–-N were significantly correlated with soil temperature at 5 cm, and soil DOC and DON were significantly correlated with soil water contents.