Salinity and high pH reduce denitrification rates by inhibiting denitrifying gene abundance in a saline-alkali soil.

Denitrification, as the main nitrogen (N) removal process in farmland drainage ditches in coastal areas, is significantly affected by saline-alkali conditions. To elucidate the effects of saline-alkali conditions on denitrification, incubation experiments with five salt and salt-alkali gradients and three nitrogen addition levels were conducted in a saline-alkali soil followed by determination of denitrification rates and the associated functional genes (i.e., nirK/nirS and nosZ Clade I) via N2/Ar technique in combination with qPCR. The results showed that denitrification rates were significantly decreased by 23.83-50.08%, 20.64-57.31% and 6.12-54.61% with salt gradient increasing from 1 to 3‰, 8‰, and 15‰ under 0.05‰, 0.10‰ and 0.15‰ urea addition conditions, respectively. Similarly, denitrification rates were significantly decreased by 44.57-63.24% with an increase of the salt-alkali gradient from 0.5 to 8‰. The abundance of nosZ decreased sharply in the saline condition, while a high salt level significantly decreased the abundance of nirK and nirS. In addition, the increase of nitrogen concentration attenuated the reduction of nirK, nirS and nosZ gene abundance. Partial least squares regression (PLSR) models demonstrated that salinity, dissolved oxygen (DO) in the overlying water, N concentration, and denitrifying gene abundance were key determinants of the denitrification rate in the saline environment, while pH was an additional determinant in the saline-alkali environment. Taken together, our results suggest that salinity and high pH levels decreased the denitrification rates by significantly inhibiting the abundance of the denitrifying genes nirK, nirS, and nosZ, whereas increasing nitrogen concentration could alleviate this effect. Our study provides helpful information on better understanding of reactive N removal and fertilizer application in the coastal areas.

Comparison of IDW, cokriging and ARMA for predicting spatiotemporal variability of soil salinity in a gravel-sand mulched jujube orchard.

Information about the spatiotemporal variability of soil salinity is important for managing salinization in gravel-sand mulched fields. We used inverse distance weighting (IDW) and cokriging to model the spatial variability of soil salinity from 2013 to 2016 and used an autoregressive moving-average (ARMA) model time series to analyze the temporal variability. The objectives of this paper are (a) to compare IDW and cokriging for predicting salinity in deep soil layers from surface data, thus finding a more appropriate method to model the spatial variability of soil salinity, and, using ARMA time series, (b) to identify one or a few sampling points, where soil salt content is the most temporally stable, to increase sampling efficiency or decrease cost and to estimate the overall soil salt content of a field. The IDW interpolation was more accurate than cokriging when using surface salt content to estimate the content in deep layers; so, we used IDW to interpolate the data and draw spatial distribution maps of salt content. Salinity in the 0-10 cm layer gradually decreased with the amount of gravel-sand mulching, from 1.02 to 0.7 g/kg over four years, and increased with depth. ARMA was accurate when using sample dates to predict soil salinity in the time series, and the model was more stable. The stability of the salt spatial patterns over time and along the soil profile allowed us to identify a location representative of the field-mean salt content, with mean relative error ranging between 0.56 and 2.19%. The monitoring of soil salt from a few observations is thus a valuable tool for practitioners and will aid the management of soil salt in gravel-sand-mulched fields in arid regions, with a range of potential applications beyond the framework of monitoring salinity.

Effect of various concentrations of superabsorbent polymers on soil particle-size distribution and evaporation with sand mulching.

Superabsorbent polymers (SAPs) are type of hydrogels capable to swell and absorb a large amount of water, but easily decomposed and oxidized by the air. We used electron-microscopic imaging in an indoor simulation with sand mulching to test the effects of various SAP concentrations on controlling evaporation and salt formation. The treatments were sand-mulched columns containing 0, 0.1, 0.2, 0.5 and 1.0% SAP. The soil particle pores were from dense to sparse and the corresponding fractal dimension decreased as SAP concentration increased. SAP concentration was correlated negatively with fractal dimension, clay-particle fraction and silt-volume fraction. And it showed a positive correlation with sand volume fraction. SAP concentration significantly affected the particle-size distribution. Water-storage capacity increased in each column layer (five 8-cm layers) at the same infiltration depth. Evaporation decreased the water content of each layer. Sand mulching combined with the SAP decreased evaporation in each layer relative to the control, which retained more water and decreased the accumulation of surface salt in the order 1.0% > 0.5% > 0.2% > 0.1% > 0. Salt migrated at 0-30 cm with sand mulching but 0-25 cm with sand mulching and SAP amendment. The decrease in salt accumulation was most effective at a SAP concentration of 0.2%.