Shuping Qin

Tillage Effects on Intracellular and Extracellular Soil Urease Activities Determined by an Improved Chloroform Fumigation Method

The intracellular urease activity in soils determined by the chloroform fumigation method is underestimated because of urease degradation by soil protease and denaturation by chloroform. The objectives of this study were to correct this underestimation and to investigate the responses of intracellular, extracellular, and total urease activities to no-tillage, reduced tillage, and moldboard plowing treatments. Surface layer soils (0-20 cm) after 8-year different tillage treatments were collected. First, the soil samples were fumigated with ethanol-free chloroform in the presence of a protease inhibitor. Subsequently, a urease addition experiment was conducted to determine what percentage of soil urease was denatured during the fumigation procedure. Our results showed that the corrected intracellular urease activities in soils under no-tillage, reduced tillage, and moldboard plowing accounted for 58.2%, 62.3%, and 51.7% of the total urease activity, respectively. Both no-tillage and reduced tillage significantly increased the intracellular, extracellular, and total urease activities compared with moldboard plowing. The ratio between intracellular and extracellular urease activities was not significantly different between no-tillage and reduced tillage probably because the soil ammonia content was similar under these treatments. The three urease activity pools were significantly correlated with soil organic C, microbial biomass C and N, and available P. These results indicate that both the intracellular and extracellular urease activities were sensitive indicators for monitoring soil quality under different tillage practices.

Denitrification Rates and Controlling Factors for Accumulated Nitrate in the 0-12 m Intensive Farmlands: a Case Study in the North China Plain

Abstract Denitrification in subsoil (to a depth of 12 m) is an important mechanism to reduce nitrate ( NO 3 − ) leaching into groundwater. However, regulating mechanisms of subsoil denitrification, especially those in the deep subsoil beneath the crop root zone, have not been well documented. In this study, soil columns of 0–12 m depth were collected from intensively farmed fields in the North China Plain. The fields had received long-term nitrogen (N) fertilizer inputs at 0 (N0), 200 (N200) and 600 (N600) kg N ha−1 year−1. Main soil properties related to denitrification, i.e., soil water content, NO 3 − , dissolved organic carbon (DOC), soil organic carbon (SOC), pH, denitrifying enzyme activity (DEA), and anaerobic denitrification rate (ADR), were determined. Statistical comparisons among the treatments were performed. The results showed that NO 3 − was more heavily accumulated in the entire soil profile of the N600 treatment, compared to the N0 and N200 treatments. The SOC, DOC, and ADR decreased with increasing soil depth in all treatments, whereas considerable DEA was observed throughout the subsoil. The long-term fertilizer rates affected ADR only in the upper 4 m soil layers. The ADRs in the N200 and N600 treatments were significantly correlated with DOC. Multiple regression analysis indicated that DOC rather than DEA was the key factor regulating denitrification beneath the root zone. Additional research is required to determine if carbon addition into subsoil can be a promising approach to enhance NO 3 − denitrification in the subsoil and consequently to mitigate groundwater NO 3 − contamination in the intensive farmlands.

Differentiating Intracellular from Extracellular Alkaline Phosphatase Activity in Soil by Sonication

Differentiating intracellular from extracellular enzyme activity is important in soil enzymology, but not easy. Here, we report on an adjusted sonication method for the separation of intracellular from extracellular phosphatase activity in soil. Under optimal sonication conditions [soil:water ratio  =  1/8 (w/v) and power density  =  15 watt ml-1], the activity of alkaline phosphomonoesterase (phosphatase) in a Haplic Cambisol soil increased with sonication time in two distinct steps. A first plateau of enzyme activity was reached between 60 and 100 s, and a second higher plateau after 300 s. We also found that sonication for 100 s under optimal conditions activated most (about 80%) of the alkaline phosphatase that was added to an autoclaved soil, while total bacteria number was not affected. Sonication for 300 s reduced the total bacteria number by three orders of magnitude but had no further effects on enzyme activity. Our results indicate that the first plateau of alkaline phosphatase activity was derived from extracellular enzymes attached to soil particles, and the second plateau to the combination of extracellular and intracellular enzymes after cell lysis. We conclude that our adjusted sonication method may be an alternative to the currently used physiological and chloroform-fumigation methods for differentiating intracellular from extracellular phosphatase activity in soil. Further testing is needed to find out whether this holds for other soil types.

Perturbation-free measurement of in situ di-nitrogen emissions from denitrification in nitrate-rich aquatic ecosystems

Increased production of reactive nitrogen (Nr) from atmospheric di-nitrogen (N2) has greatly contributed to increased food production. However, enriching the biosphere with Nr has also caused a series of negative effects on global ecosystems, especially aquatic ecosystems. The main pathway converting Nr back into the atmospheric N2 pool is the last step in the denitrification process. Despite several attempts, there is still a need for perturbation-free methods for measuring in situ N2 fluxes from denitrification in aquatic ecosystems at the field scale. Such a method is needed to comprehensively quantify the N2 fluxes from aquatic ecosystems. Here we observed linear relationships between the δ15N-N2O signatures and the logarithmically transformed N2O/(N2+N2O) emission ratios. Through independent measurements, we verified that the perturbation-free N2 flux from denitrification in nitrate-rich aquatic ecosystems can be inferred from these linear relationships. Our method allowed the determination of field-scale in situ N2 fluxes from nitrate-rich aquatic ecosystems both with and without overlaying water. The perturbation-free in situ N2 fluxes observed by the new method were almost one order of magnitude higher than those by the sediment core method. The ability of aquatic ecosystems to remove Nr may previously have been severely underestimated.