Long-Term Manure Amendments Enhance Soil Aggregation and Carbon Saturation of Stable Pools in North China Plain

Organic amendment is considered as an effective way to increase soil organic carbon （SOC） stock in croplands. To better understand its potential for SOC sequestration, whether SOC saturation could be observed in an intensive agricultural ecosystem receiving long-term composted manure were examined. Different SOC pools were isolated by physical fractionation techniques ofa Cambisol soil under a long-term manure experiment with wheat-maize cropping in North China Plain. A field experiment was initiated in 1993, with 6 treatments including control （i.e., without fertilization）, chemical fertilizer only, low rate of traditional composted manure （7.5 t ha-h）, high rate of traditional composted manure （15 t ha-~）, low rate ofbio-composted manure （7.5 t ha-h） and high rate of bio-composted manure （15 t ha-h）. The results showed that consecutive （for up to 20 years） composted manure amendments significantly improved soil macro-aggregation, aggregate associated SOC concentration, and soil structure stability. In detail, SOC concentration in the sand-sized fraction （〉53 ~tm） continued to increase with manure application rate, while the silt （2-53 I.tm） and clay （〈2 ~tm） particles showed no further increase with greater C inputs, exhibiting the C saturation. Further physical separation of small macro-aggregates （250-2 000 tam） into subpools showed that the non-protected coarse particulate organic matter （cPOM, 〉250 pro） was the fraction in which SOC continued to increase with increasing manure application rate. In contrast, the chemical and physical protected C pools （i.e., micro-aggregates and silt-clay occluded in the small macro- aggregates） exhibited no additional C sequestration when the manure application rate was increased. It can be concluded that repeated manure amendments can increase soil macro-aggregation and lead to the increase in relatively stable C pools, showing hierarchical saturation behavior in the intensive cropping system of North China Plain.

Simulated NH_4^+-N Deposition Inhibits CH_4 Uptake and Promotes N_2O Emission in the Meadow Steppe of Inner Mongolia, China

Few studies are conducted to quantify the effects of enhanced N deposition on soil nitrous oxide （N2O） emission and methane （CH4） uptake in the meadow steppe of Inner Mongolia, China. A two-year field experiment was conducted to assess the effects of nitrogen （N） deposition rates （0, 10, and 20 kg N ha-1 year-1 as （NH4）2SO4） on soil N2O and CH4 fluxes. The seasonal and diurnal variations of soil N2O and CH4 fluxes were determined using the static chamber-gas chromatography method during the two growing seasons of 2008 and 2009. Soil temperature, moisture and mineral N （NH4＋-N and NO3-N） concentration were simultaneously measured. Results showed that low level of （NH4）2SO4 （10 kg N ha-1 year-1） did not significantly affect soil CH4 and N20 fluxes and other variables. High level of （NH4）2SO4 （20 kg N ha-1 year-1） significantly increased soil NO3-N concentration by 24.1% to 35.6%, decreased soil CH4 uptake by an average of 20.1%, and significantly promoted soil N2O emission by an average of 98.2%. Soil N2O emission responded more strongly to the added N compared to CH4 uptake. However, soil CH4 fluxes were mainly driven by soil moisture, followed by soil NO3--N concentration. Soil N2O fluxes were mainly driven by soil temperature, followed by soil moisture. Soil inorganic N availability was a key integrator of soil CH4 uptake and N2O emission. These results suggest that the changes of availability of inorganic N induced by the increased N deposition in soil may affect the CH4 and N2O fluxes in the cold semi-arid meadow steppe over the short term.

Effects of rye grass coverage on soil loss from loess slopes

Vegetative coverage is commonly used to reduce urban slope soil erosion.Laboratory experimental study on soil erosion under grass covered slopes is conventionally time and space consuming.In this study,a new method is suggested to study the influences of vegetation coverage on soil erosion from a sloped loess surface under three slope gradients of 51,151,and 251;four rye grass coverages of 0%,25%,50%,and 75%;and three rainfall intensities of 60,90,and 120 mm/h with a silt-loamy loess soil.Rye grasses were planted in the field with the studied soil before being transplanted into a laboratory flume.Grass was allowed to resume growth for a period before the rain simulation experiment.Results showed that the grass cover reduced soil erosion by 63.90%to 92.75%and sediment transport rate by 80.59%to 96.17%under different slope gradients and rainfall intensities.The sediment concentration/sediment transport rate from bare slope was significantly higher than from a grass-covered slope.The sediment concentration/transport rate from grass-covered slopes decreased linearly with grass coverage and increased with rainfall intensity.The sediment concentration/transport rate from the bare slope increased as a power function of slope and reached the maximum value at the gradient of about 251,whereas that from grass-covered slope increased linearly and at much lower levels.The results of this study can be used to estimate the effect of vegetation on soil erosion from loess slopes.