Revealing the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation.

It has been recently demonstrated that free DNA tracers have the potential in tracing water flow and contaminant transport through the vadose zone. However, whether the free DNA tracer can be used in flood irrigation area to track water flow and solute/contaminant transport is still unclear. To reveal the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation, we tested the fate and transport behavior of surface applied free DNA tracers through packed saturated sandy soil columns with a 10 cm water head mimicking flood irrigation. From the experimental breakthrough curves and by fitting a two-site kinetic sorption model (R2 = 0.83-0.91 and NSE = 0.79-0.89), adsorption/desorption rates could be obtained and tracer retention profiles could be simulated. Together these results revealed that 1) the adsorption of free DNA was dominantly to clay particles in the soil, which took up 1.96 % by volume, but took up >97.5 % by surface area and densely cover the surface of sand particles; and 2) at a pore water pH of 8.0, excluding the 4.9 % passing through and 3.1 % degradation amount, the main retention mechanisms in the experimental soil were ligand exchange (42.0 %), Van der Waals interactions (mainly hydrogen bonds), electrostatic forces and straining (together 44.7 %), and cation bridge (5.3 %). To our knowledge, this study is the first to quantify the contribution of each of the main retention mechanisms of free synthetic DNA tracers passing through soil. Our findings could facilitate the application of free DNA tracer to trace vadose zone water flow and solute/contaminant transport under flood irrigation and other infiltration conditions.

Roles of sulfur compounds in growth and alkaline phosphatase activities of Microcystis aeruginosa under phosphorus deficiency stress.

Microcystis aeruginosa is one of the most common algae found in eutrophicated water bodies. Alkaline phosphatase (AKP) can be produced by Microcystis aeruginosa to utilize organic phosphates under phosphorus deficiency stress, thereby AKP can be regarded as an important indicator for algal growth. Sulfur compounds are ubiquitous in waters, while investigation on the interactions between sulfur compounds and Microcystis aeruginosa is limited. In this work, we introduced 33 types of sulfur compounds to culture Microcystis aeruginosa, and the results demonstrated that algal growth is positively related to AKP activities. Toxicity of organic sulfur compounds was further evaluated using Toxicity Estimation Software Tool based on quantitative structure-activity relationship prediction. The algal growth results exhibited strong correlation to the toxicity endpoints suggesting the organic sulfur compounds inhibits the algal growth as toxic matters. K-means cluster analyses have been carried out subsequently via Python based on the results of algal growth and AKP activities of each sample and statistically, the sulfur compounds can be adequately clustered into 2 groups. According to clustering results, sulfonic acids exhibit low toxicity while sulfur amino acids can be considered as more toxic compounds. Graphical abstract Varied sulfur compounds (33 types) were investigated to find out the interactions between them and Microcystis aeruginosa, a common alga. K-means cluster and correlation analyses demonstrate that algal growth and alkaline phosphatase activities exhibited strong correlation to the predicted toxicity endpoints.

Abiotic carbonate dissolution traps carbon in a semiarid desert.

It is generally considered that desert ecosystems release CO2 to the atmosphere, but recent studies in drylands have shown that the soil can absorb CO2 abiotically. However, the mechanisms and exact location of abiotic carbon absorption remain unclear. Here, we used soil sterilization, (13)CO2 addition, and detection methods to trace (13)C in the soil of the Mu Us Desert, northern China. After (13)CO2 addition, a large amount of (13)CO2 was absorbed by the sterilised soil, and (13)C was found enriched both in the soil gaseous phase and dissolved inorganic carbon (DIC). Further analysis indicated that about 79.45% of the total (13)C absorbed by the soil was trapped in DIC, while the amount of (13)C in the soil gaseous phase accounted for only 0.22% of the total absorbed (13)C. However, about 20.33% of the total absorbed (13)C remained undetected. Our results suggest that carbonate dissolution might occur predominately, and the soil liquid phase might trap the majority of abiotically absorbed carbon. It is possible that the trapped carbon in the soil liquid phase leaches into the groundwater; however, further studies are required to support this hypothesis.

Patterns and possible mechanisms of soil CO2 uptake in sandy soil.

It has been reported that soils in drylands can absorb CO2, although the patterns and mechanisms of such a process remain under debate. To address this, we investigated the relationships between soil CO2 flux and meteorological factors and soil properties in Northwest China to reveal the reasons for "anomalous" soil CO2 flux in a desert ecosystem. Soil CO2 flux increased significantly and exponentially with surficial turbulence at the diel scale under dry conditions (P<0.05), whereas the relationship under wet conditions was insignificant. Furthermore, soil CO2 flux demonstrated remarkable negative correlation with soil air pressure (P<0.05) in both dry and wet conditions. Analysis considering Henry's Law indicated that soil water content was insufficient to dissolve the absorbed CO2 in dry conditions, but was sufficient in wet conditions. The concentration of soil HCO3(-) in the morning was higher than in the evening in dry conditions, but this pattern was reversed in wet conditions. These results imply that CO2 outgassing induced by turbulence, expansion of soil air, CO2 effusion from soil water, and carbonate precipitation during daytime can explain the abiotic diurnal CO2 release. Moreover, CO2 pumping from the atmosphere into the soil, caused mainly by carbonate dissolution, can account for nocturnal CO2 absorption in dry conditions. The abiotic soil CO2 flux pattern (CO2 absorption throughout the diel cycle) in wet conditions can be attributed to downward mass flow of soil CO2 and intensified soil air shrinkage, CO2 dissolving in soil water, and carbonate dissolution. These results provide a basis for determining the location of abiotic fixed carbon within soils in desert ecosystems.

Effect of vegetation rehabilitation on soil carbon and its fractions in Mu Us Desert, northwest China.

Although vegetation rehabilitation on semi-arid and arid regions may enhance soil carbon sequestration, its effects on soil carbon fractions remain uncertain. We carried out a study after planting Artemisia ordosica (AO, 17 years), Astragalus mongolicum (AM, 5 years), and Salix psammophila (SP, 16 years) on shifting sand land (SL) in the Mu Us Desert, northwest China. We measured total soil carbon (TSC) and its components, soil inorganic carbon (SIC) and soil organic carbon (SOC), as well as the light and heavy fractions within soil organic carbon (LF-SOC and HF-SOC), under the SL and shrublands at depths of 100 cm. TSC stock under SL was 27.6 Mg ha(-1), and vegetation rehabilitation remarkably elevated it by 40.6 Mgha(-1), 4.5 Mgha(-1), and 14.1 Mgha(-1) under AO, AM and SP land, respectively. Among the newly formed TSC under the three shrublands, SIC, LF-SOC and HF-SOC accounted for 75.0%, 10.7% and 13.1% for AO, respectively; they made up 37.0%, 50.7% and 10.6% for AM, respectively; they occupied 68.6%, 18.8% and 10.0% for SP, respectively. The accumulation rates of TSC within 0-100 cm reached 238.6 g m(-2) y(-1), 89.9 g m(-2) y(-1) and 87.9 g m(-2) y(-1) under AO, AM and SP land, respectively. The present study proved that the accumulation of SIC considerably contributed to soil carbon sequestration, and vegetation rehabilitation on shifting sand land has a great potential for soil carbon sequestration.

Impact of environmental factors and biological soil crust types on soil respiration in a desert ecosystem.

The responses of soil respiration to environmental conditions have been studied extensively in various ecosystems. However, little is known about the impacts of temperature and moisture on soils respiration under biological soil crusts. In this study, CO2 efflux from biologically-crusted soils was measured continuously with an automated chamber system in Ningxia, northwest China, from June to October 2012. The highest soil respiration was observed in lichen-crusted soil (0.93 ± 0.43 µmol m-2 s-1) and the lowest values in algae-crusted soil (0.73 ± 0.31 µmol m-2 s-1). Over the diurnal scale, soil respiration was highest in the morning whereas soil temperature was highest in the midday, which resulted in diurnal hysteresis between the two variables. In addition, the lag time between soil respiration and soil temperature was negatively correlated with the soil volumetric water content and was reduced as soil water content increased. Over the seasonal scale, daily mean nighttime soil respiration was positively correlated with soil temperature when moisture exceeded 0.075 and 0.085 m3 m-3 in lichen- and moss-crusted soil, respectively. However, moisture did not affect on soil respiration in algae-crusted soil during the study period. Daily mean nighttime soil respiration normalized by soil temperature increased with water content in lichen- and moss-crusted soil. Our results indicated that different types of biological soil crusts could affect response of soil respiration to environmental factors. There is a need to consider the spatial distribution of different types of biological soil crusts and their relative contributions to the total C budgets at the ecosystem or landscape level.

Influence of disturbance on soil respiration in biologically crusted soil during the dry season.

Soil respiration (Rs) is a major pathway for carbon cycling and is a complex process involving abiotic and biotic factors. Biological soil crusts (BSCs) are a key biotic component of desert ecosystems worldwide. In desert ecosystems, soils are protected from surface disturbance by BSCs, but it is unknown whether Rs is affected by disturbance of this crust layer. We measured Rs in three types of disturbed and undisturbed crusted soils (algae, lichen, and moss), as well as bare land from April to August, 2010, in Mu Us desert, northwest China. Rs was similar among undisturbed soils but increased significantly in disturbed moss and algae crusted soils. The variation of Rs in undisturbed and disturbed soil was related to soil bulk density. Disturbance also led to changes in soil organic carbon and fine particles contents, including declines of 60-70% in surface soil C and N, relative to predisturbance values. Once BSCs were disturbed, Q 10 increased. Our findings indicate that a loss of BSCs cover will lead to greater soil C loss through respiration. Given these results, understanding the disturbance sensitivity impact on Rs could be helpful to modify soil management practices which promote carbon sequestration.