Deep ploughing increases agricultural soil organic matter stocks.

Subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC concentrations. However, measures for enhancing SOC storage commonly focus on topsoils. This study assessed the long-term storage and stability of SOC in topsoils buried in arable subsoils by deep ploughing, a globally applied method for breaking up hard pans and improving soil structure to optimize crop growing conditions. One effect of deep ploughing is translocation of SOC formed near the surface into the subsoil, with concomitant mixing of SOC-poor subsoil material into the 'new' topsoil. Deep-ploughed croplands represent unique long-term in situ incubations of SOC-rich material in subsoils. In this study, we sampled five loamy and five sandy soils that were ploughed to 55-90 cm depth 35-50 years ago. Adjacent, similarly managed but conventionally ploughed subplots were sampled as reference. The deep-ploughed soils contained on average 42 ± 13% more SOC than the reference subplots. On average, 45 years after deep ploughing, the 'new' topsoil still contained 15% less SOC than the reference topsoil, indicating long-term SOC accumulation potential in the topsoil. In vitro incubation experiments on the buried sandy soils revealed 63 ± 6% lower potential SOC mineralisation rates and also 67 ± 2% lower SOC mineralisation per unit SOC in the buried topsoils than in the reference topsoils. Wider C/N ratio in the buried sandy topsoils than in the reference topsoils indicates that deep ploughing preserved SOC. The SOC mineralisation per unit SOC in the buried loamy topsoils was not significantly different from that in the reference topsoils. However, 56 ± 4% of the initial SOC was preserved in the buried topsoils. It can be concluded that deep ploughing contributes to SOC sequestration by enlarging the storage space for SOC-rich material.

Identification of soil contamination hotspots with veterinary antibiotics using heavy metal concentrations and leaching data--a field study in China.

In regions with high livestock densities, the usage of antibiotics and metals for veterinary purposes or as growth promoters poses a risk in manured soils. We investigated to which degree the concentrations and depth distributions of Cu, Zn, Cr and As could be used as a tracer to discover contaminations with sulfonamides, tetracyclines and fluoroquinolones. Besides, we estimated the potential vertical translocation of antibiotics and compared the results to measured data. In the peri-urban region of Beijing, China, soil was sampled from agricultural fields and a dry riverbed contaminated by organic waste disposal. The antibiotic concentrations reached 110 µg kg(-1) sulfamethazine, 111 µg kg(-1) chlortetracycline and 62 µg kg(-1) enrofloxacin in the topsoil of agricultural fields. Intriguingly, total concentrations of Cu, Zn, Cr and As were smaller than 65, 130, 36 and 10 mg kg(-1) in surface soil, respectively, therewith fulfilling Chinese quality standards. Correlations between sulfamethazine concentrations and Cu or Zn suggest that in regions with high manure applications, one might use the frequently existing monitoring data for metals to identify potential pollution hotspots for antibiotics in topsoils. In the subsoils, we found sulfamethazine down to ≥2 m depth on agricultural sites and down to ≥4 m depth in the riverbed. As no translocation of metals was observed, subsoil antibiotic contamination could not be predicted from metal data. Nevertheless, sulfonamide stocks in the subsoil could be estimated with an accuracy of 35-200 % from fertilisation data and potential leaching rates. While this may not be sufficient for precise prediction of antibiotic exposure, it may very well be useful for the pre-identification of risk hotspots for subsequent in-depth assessment studies.

Sources of nitrous and nitric oxides in paddy soils: nitrification and denitrification.

Rice-paddies are regarded as one of the main agricultural sources of N 2O and NO emissions. To date, however, specific N2O and NO production pathways are poorly understood in paddy soils. (15)N-tracing experiments were carried out to investigate the processes responsible for N2O and NO production in two paddy soils with substantially different soil properties. Laboratory incubation experiments were carried out under aerobic conditions at moisture contents corresponding to 60% of water holding capacity. The relative importance of nitrification and denitrification to the flux of N2O was quantified by periodically measuring and comparing the enrichments of the N2O, NH(+)4-N and NO(-)3-N pools. The results showed that both N2O and NO emission rates in an alkaline paddy soil with clayey texture were substantially higher than those in a neutral paddy soil with silty loamy texture. In accordance with most published results, the ammonium N pool was the main source of N2O emission across the soil profiles of the two paddy soils, being responsible for 59.7% to 97.7% of total N2O emissions. The NO(-)3-N pool of N2O emission was relatively less important under the given aerobic conditions. The rates of N2O emission from nitrification (N2On) among different soil layers were significantly different, which could be attributed to both the differences in gross N nitrification rates and to the ratios of nitrified N emitted as N2O among soil layers. Furthermore, NO fluxes were positively correlated with the changes in gross nitrification rates and the ratios of NO/N2O in the two paddy soils were always greater than one (from 1.26 to 6.47). We therefore deduce that, similar to N2O, nitrification was also the dominant source of NO in the tested paddy soils at water contents below 60% water holding capacity.

Potentially toxic elements in the environment - a review of sources, sinks, pathways and mitigation measures.

Pollution of ecosystems with potentially toxic elements (PTEs) has become a global problem with serious consequences for public health. The PTEs are hazardous to humans owing to their longevity, toxicity, and ability to accumulate in the biotic environment. As most PTEs cannot be degraded microbially or chemically, they can persist in soils for a long time. Besides posing a threat to landsphere, they may be transported to surrounding environmental spheres through movement of water, atmospheric circulation, and biological transmission. This can severely affect the ecological equilibrium. Accumulation of PTEs in soils pose serious health hazards to higher organisms leading to various diseases and disorders and significant relationships exist between the occurrence of PTEs and the toxic effects in humans. In natural soils, PTEs accumulate due to weathering of rocks and ores. Furthermore, locally or regionally significant accumulation of PTEs in soils may occur from industrial goods, pesticides and paints, municipal and industrial waste, fertilizer application, mining activities and atmospheric deposition. In response to the growing need to address PTE contamination, remediation methods have been developed employing mechanical, physico-chemical or biological based technologies. In this review, we discuss sources, sinks, pathways and mitigation measures related to natural and anthropogenic PTEs. We focus on As, Cd, Cr, Hg and Pb which are highly toxic and perform no physiological functions in biota. Further, these are the most widely studied PTEs.

Reactive nitrogen compounds and their influence on human health: an overview.

Nitrogen (N) is a critical component of food security, economy and planetary health. Human production of reactive nitrogen (Nr) via Haber-Bosch process and cultivation-induced biological N2 fixation (BNF) has doubled global N cycling over the last century. The most important beneficial effect of Nr is augmenting global food supplies due to increased crop yields. However, increased circulation of Nr in the environment is responsible for serious human health effects such as methemoglobinemia ("blue baby syndrome") and eutrophication of coastal and inland waters. Furthermore, ammonia (NH3) emission mainly from farming and animal husbandary impacts not only human health causing chronic lung disease, inflammation of human airways and irritation of eyes, sinuses and skin but is also involved in the formation of secondary particulate matter (PM) that plays a critical role in environment and human health. Nr also affects human health via global warming, depletion of stratospheric ozone layer resulting in greater intensity of ultra violet B rays (UVB) on the Earth's surface, and creation of ground-level ozone (through reaction of NO2 with O2). The consequential indirect human health effects of Nr include the spread of vector-borne pathogens, increased incidence of skin cancer, development of cataracts, and serious respiratory diseases, besides land degradation. Evidently, the strategies to reduce Nr and mitigate adverse environmental and human health impacts include plugging pathways of nitrogen transport and loss through runoff, leaching and emissions of NH3, nitrogen oxides (NO x ), and other N compounds; improving fertilizer N use efficiency; reducing regional disparity in access to N fertilizers; enhancing BNF to decrease dependence on chemical fertilizers; replacing animal-based proteins with plant-based proteins; adopting improved methods of livestock raising and manure management; reducing air pollution and secondary PM formation; and subjecting industrial and vehicular NO x emission to pollution control laws. Strategic implementation of all these presents a major challenge across the fields of agriculture, ecology and public health. Recent observations on the reduction of air pollution in the COVID-19 lockdown period in several world regions provide an insight into the achievability of long-term air quality improvement. In this review, we focus on complex relationships between Nr and human health, highlighting a wide range of beneficial and detrimental effects.

Trade-offs of gaseous emissions from soils under vegetable, wheat-maize and apple orchard cropping systems applied with digestate: An incubation study.

Land application of digestate from anaerobic digestion causes various gaseous emissions. A soil core incubation experiment was carried out in the laboratory to investigate the trade-offs of NH3, N2O and CH4 emissions from soils collected from vegetable, arable and orchard cropping systems. Digestate derived from liquid cattle manure was applied to the soil cores through the surface (SA) and incorporation application (IA) methods under three soil moisture conditions (40%, 60%, and 80% water-filled pore space, WFPS). Gaseous emissions from vegetable soil were significantly greater (P< .05) than those from soils under the other two cropping systems under similar conditions, particularly under a high moisture condition. The greenhouse gas emissions (GHG, in term of CO2-equivalents) of all soils increased with the increasing soil moisture contents, mainly due to rapidly increasing N2O emissions. Trade-offs in the emissions of these three gases were observed between SA and IA. As expected, SA was characterized by greater NH3 and CH4 but lower N2O emissions compared to IA. The increase in GHG under IA could be offset only somewhat by the reduced NH3 (and this reduced indirect N2O) and CH4 emissions under lower moisture conditions (<60% WFPS), which indicates a requirement for other strategies to control gaseous emissions from wet soils applied with digestate. In conclusion, an environmentally friendly strategy for digestate application should consider the soil moisture, types of soils and application methods, and all the presented suggestions need to be verified in the field in the future.Implications: This study shows that digestate incorporation can decrease NH3 but increase GHG emissions verse the surface application method, where the increased GHG could only be offset by the NH3 reductions at relatively dry soil condition, indicating an urgent requirement to mitigating GHG emissions under moist soil condition.

Stability of buried carbon in deep-ploughed forest and cropland soils - implications for carbon stocks.

Accumulation of soil organic carbon (SOC) may play a key role in climate change mitigation and adaptation. In particular, subsoil provides a great potential for additional SOC storage due to the assumed higher stability of subsoil SOC. The fastest way in which SOC reaches the subsoil is via burial, e.g. via erosion or deep ploughing. We assessed the effect of active SOC burial through deep ploughing on long-term SOC stocks and stability in forest and cropland subsoil. After 25-48 years, deep-ploughed subsoil contained significantly more SOC than reference subsoils, in both forest soil (+48%) and cropland (+67%). However, total SOC stocks down to 100 cm in deep-ploughed soil were greater than in reference soil only in cropland, and not in forests. This was explained by slower SOC accumulation in topsoil of deep-ploughed forest soils. Buried SOC was on average 32% more stable than reference SOC, as revealed by long-term incubation. Moreover, buried subsoil SOC had higher apparent radiocarbon ages indicating that it is largely isolated from exchange with atmospheric CO2. We concluded that deep ploughing increased subsoil SOC storage and that the higher subsoil SOC stability is not only a result of selective preservation of more stable SOC fractions.