Effects of different tillage and straw retention practices on soil aggregates and carbon and nitrogen sequestration in soils of the northwestern China

Soil tillage and straw retention in dryland areas may affect the soil aggregates and the distribution of total organic carbon. The aims of this study were to establish how different tillage and straw retention practices affect the soil aggregates and soil organic carbon(SOC) and total nitrogen(TN) contents in the aggregate fractions based on a long-term(approximately 15 years) field experiment in the semi-arid western Loess Plateau, northwestern China. The experiment included four soil treatments, i.e., conventional tillage with straw removed(T), conventional tillage with straw incorporated(TS), no tillage with straw removed(NT) and no tillage with straw retention(NTS), which were arranged in a complete randomized block design. The wet-sieving method was used to separate four size fractions of aggregates, namely, large macroaggregates(LA, >2000 μm), small macroaggregates(SA, 250–2000 μm), microaggregates(MA, 53–250 μm), and silt and clay(SC, <53 μm). Compared to the conventional tillage practices(including T and TS treatments), the percentages of the macroaggregate fractions(LA and SA) under the conservation tillage practices(including NT and NTS treatments) were increased by 41.2%–56.6%, with the NTS treatment having the greatest effect. For soil layers of 0–5, 5–10 and 10–30 cm, values of the mean weight diameter(MWD) under the TS and NTS treatments were 10.68%, 13.83% and 17.65%, respectively. They were 18.45%, 19.15% and 14.12% higher than those under the T treatment, respectively. The maximum contents of the aggregate-associated SOC and TN were detected in the SA fraction, with the greatest effect being observed for the NTS treatment. The SOC and TN contents were significantly higher under the NTS and TS treatments than under the T treatment. Also, the increases in SOC and TN levels were much higher in the straw-retention plots than in the straw-removed plots. The macroaggregates(including LA and SA fractions) were the major pools for SOC and TN, regardless of tillage practices, storing 3.25–6.81 g C/kg soil and 0.34–0.62 g N/kg soil. Based on the above results, we recommend the NTS treatment as the best option to boost soil aggregates and to reinforce carbon and nitrogen sequestration in soils in the semi-arid western Loess Plateau of northwestern China.

Evolution of soil and water conservation in rain-fed areas of China

Rain-fed(dryland)farming is an ancient agricultural production system in China.It occurs widely across almost the whole country,especially in the Northwest and North China.The semi-arid Loess Plateau is the most important region of rain-fed farming in China,but unfortunately,soil erosion on the Loess Plateau area is the highest in China,and indeed amongst the highest in the world.This highlights the necessity for developing practices that can reduce soil and water erosion,improve soil water use efficiency,improve crop productivity,and reduce rural poverty in the region.Many techniques of soil and water conservation are being used in rain-fed areas of China,including such systems as mulch,ridge and furrow systems.The Appendix describes a unique system of soil and water conservation,called Shatian.Modern research on conservation tillage(No Till),although essential for reducing erosion,increasing crop productivity,and ameliorating poverty,is just beginning in China.Modern conservation tillage research started in the1990s’with support from Australia and other countries.The procedures,however,were modified to be in accord with local conditions and prevailing farmer experiences.With 10 years of experimentation,results show that the most successful conservation practice on the Western Loess Plateau is no till with stubble retention.This technique helps to conserve soil water,increases soil organic carbon,improves soil structure and water infiltration,reduces soil and water erosion,and improves crop productivity and sustainability of rain-fed farming systems.However,its adoption rate remains low due to barriers such as traditional attitude,insufficient rural extension,and so forth.