Distribution characteristics of organic carbon and total nitrogen in dry farmland soil aggregates under different tillage methods in the Loess Plateau of central Gansu Province

A 12-year local experiment of spring wheat-pea rotation system was conducted in dry farmlands of the Loess Plateau in central Gansu. The aim of the study was to determine the distribution characteristics of soil organic carbon and total nitrogen in soil aggregates. The treatments consisted of four tillage practices ― conventional tillage without straw mulch (T), conventional tillage with straw incorporation (TS), no-tillage without straw mulching (NT) and no-tillage with straw mulching (NTS). The experimental set-up was a Randomized Complete Block design with three replications. The soil samples were taken at three different soil depths (0 5 cm, 5 10 cm and 10 30 cm) per plot. The results showed that the ≥0.25 mm aggregates of soil were the dominant aggregates, whose content increased with increasing soil depth. While increase in the content of ≥0.25 mm aggregates was regular, changes in the ≥5 mm, 2 5 mm, 0.25 2 mm aggregates were irregular. Compared to T treatment, NT, TS and NTS treatments increased the ≥0.25 mm aggregates content and mean weight diameter (MWD) of aggregates, and NTS had the highest effects. With increasing soil depth, MWD of all four treatments improved under pea→wheat→pea rotation sequence. However, the results under wheat→pea→wheat rotation sequence was different. MWD of T and NT treatments in three soil depths did not increase with increasing soil depth. However, MWD of TS and NTS treatments increased with increasing soil depth. Compared with T treatment, NT, TS and NTS treatments increased the contents of soil organic carbon and total nitrogen. NTS had the highest soil organic carbon and total nitrogen contents. NTS and TS were significantly higher (P < 5%) than T. The results showed that the contents of soil organic carbon and total nitrogen decreased with increasing soil depth. Compared with T treatment, NT, TS, NTS treatments improved soil organic carbon and total nitrogen in soil aggregates. NTS had the best for all the investigated soil depths. The results showed that with increasing soil depth, the contents of soil organic carbon and total nitrogen decreased in soil aggregates. Also the contents of soil organic carbon and total nitrogen decreased in soil aggregates with increasing aggregate size. The correlation between the 2 5 mm and 0.25 2 mm soil aggregates contents and their soil organic carbon content was significantly positive ― respectively with correlation coefficients of 0.493 and 0.559. The correlation between the ≥5 mm soil aggregate content and its soil organic carbon content was significantly negative ( 0.481). The correlation between the 0.25 2 mm and ≥5 mm soil aggregates contents and their total nitrogen contents was respectively significantly positive (0.590) and significantly negative ( 0.418). Therefore the size of the 0.25 2 mm soil aggregate was the major driver of soil organic and total nitrogen. The contributing rates of organic carbon and total nitrogen in different sized soil aggregates to soil organic carbon and total nitrogen in T treatment were in the order of (<0.25 mm) > (≥5 mm) > (0.25 2 mm) > (2 5 mm) in all the soil depths. The trends for the other treatments varied with different soil depths. Long-term conservation tillage increased soil stability, the ≥0.25 mm soil aggregate amount, soil aggregates mean weight diameter, soil organic carbon content and total nitrogen contents. NTS treatment enhanced amount of ≥0.25 mm soil aggregates improving soil organic carbon, total nitrogen and soil stability.