The contribution of natural and anthropogenic causes to soil acidification rates under different fertilization practices and site conditions in southern China.

Excessive application of mineral fertilizers has accelerated soil acidification in China, affecting crop production when the pH drops below a critical value. However, the contributions of natural soil acidification, induced by leaching of bicarbonate, and anthropogenic causes of soil acidification, induced by nitrogen (N) transformations and removal of base cations over acid anions, are not well quantified. In this study, we quantified soil acidification rates, in equivalents (eq) of acidity, by assessing the inputs and outputs of all major cations and anions, including calcium, magnesium, potassium, sodium, ammonium, nitrate, bicarbonate, sulphate, phosphate and chloride, for 13 long-term experimental sites in southern China. The acidification rates strongly varied among fertilizer treatments and with the addition of animal manure. Bicarbonate leaching was the dominant acid production process in calcareous soils (23 keq ha-1 yr-1) and in non-calcareous paddy soils (9.6 keq ha-1 yr-1), accounting for 80 % and 68 % of the total acid production rate, respectively. The calcareous soils were strongly buffered, and acidification led no or a limited decline in pH. In contrast, N transformations were the most important driver for soil acidification at one site with upland crops on a non-calcareous soil, accounting for 72 % of total acid production rate of 8.4 keq ha-1 yr-1. In this soil, the soil pH considerably decreased being accompanied by a substantial decline in exchangeable base cation. Reducing the N surplus decreased the acidification rate with 10 to 54 eq per kg N surplus with the lowest value occurring in paddy soils and the highest in the upland soil. The use of manure, containing base cations, partly mitigated the acidifying impact of N fertilizer inputs and crop removal, but enhanced phosphorus (P) accumulation. Combining mineral fertilizer, manure and lime in integrative management strategies can mitigate soil acidification and minimize N and P losses.

Long-term liming mitigates the positive responses of soil carbon mineralization to warming and labile carbon input.

Liming, as a common amelioration practice worldwide, has the potential to alleviate soil acidification and ensure crop production. However, the impacts of long-term liming on the temperature sensitivity (Q10) of soil organic carbon (SOC) mineralization and its response to labile C input remain unclear. To fill the knowledge gap, soil samples were collected from a long-term (∼10 years) field trial with unlimed and limed (CaO) plots. These soil samples were incubated at 15 °C and 25 °C for 42 days, amended without and with 13C-labeled glucose. Results showed that compared to the unlimed soil (3.6-8.6 mg C g-1 SOC), liming increased SOC mineralization (6.1-11.2 mg C g-1 SOC). However, liming significantly mitigated the positive response of SOC mineralization to warming, resulting in a lower Q10. Long-term liming increased bacterial richness and Shannon diversity as well as their response to warming which were associated with the decreased Q10. Furthermore, the decreased Q10 due to liming was attributed to the decreased response of bacterial oligotrophs/copiotrophs ratio, ß-glucosidase and xylosidase activities to warming. Labile C addition had a strong impact on Q10 in the unlimed soil, but only a marginal influence in the limed soil. Overall, our research highlights that acidification amelioration by long-term liming has the potential to alleviate the positive response of SOC mineralization to warming and labile C input, thereby facilitating SOC stability in agroecosystems, especially for acidic soils in subtropical regions.

Major drivers of soil acidification over 30 years differ in paddy and upland soils in China.

Elevated nitrogen (N) fertilization has largely increased crop production in China, but also increased acidification risks, thereby threatening crop yields. However, natural soil acidification due to bicarbonate (HCO3) leaching and base cation (BC) removal by crop harvest also affect soil acidity whereas the input of HCO3 and BC via fertilizers and manure counteract soil acidification. Insights in rates and drivers of soil acidification in different land use types is too limited to support crop- and site-specific mitigation strategies. In this study, we assessed the historical changes in cropland acidification rates and their drivers for the period 1985-2019 at 151 sites in a typical Chinese county with the combined nutrient and soil acidification model VSD+. VSD+ could well reproduce long-term changes in pH and in the BC concentrations of calcium, magnesium and potassium between 1985 and 2019 in non-calcareous soils. In paddy soils, the acidity production rate decreased from 1985 onwards, mainly driven by a pH-induced reduction in HCO3 leaching and N transformations. In upland soils, however, acidity production was mainly driven by N transformations and hardly changed over time. Crop BC removal by harvesting played a minor role in both paddy and upland soils, but its relative importance increased in paddy soils. The acidity input was partly neutralized by HCO3 input from fertilizers and manure, which decreased over time due to a change from ammonia bicarbonate to urea. Soil buffering by both BC and aluminium release decreased in paddy soils due to a reduction in net acidity production, while it stayed relatively constant in upland soils. We conclude that acidification management in paddy soils requires a focus on avoiding high HCO3 leaching whereas the management in upland soils should focus on balancing N with recycling organic manure and crop residues.

Deciphering Biotic and Abiotic Mechanisms Underlying Straw Decomposition and Soil Organic Carbon Priming in Agriculture Soils Receiving Long-Term Fertilizers.

Straw-related carbon (C) dynamics are central for C accrual in agro-ecosystems and should be assessed by investigating their decomposition and soil organic carbon (SOC) priming effects. Our understanding of biotic and abiotic mechanisms underpinning these two C processes, however, is still not sufficiently profound. Soils that had received organic and mineral fertilizers for 26 years were sampled for a 28 day incubation experiment to assess 13C-labeled straw decomposition and SOC priming effects. On the basis of analyzing physicochemical properties, fungal taxonomic (MiSeq sequencing) and functional (metagenomics) guilds, we quantified the contributions of biotic and abiotic attributes to straw decomposition and SOC priming. Here, we propose two distinct mechanisms underlying straw decomposition and SOC priming in agriculture soils: (i) accelerated straw mineralization in manure-treated soils was mainly driven by biotic forces, while (ii) larger SOC priming in NPK-amended soils was through abiotic regulation.

Mechanism of Al toxicity alleviation in acidic red soil by rice-straw hydrochar application and comparison with pyrochar.

In the past decade, biochar has been widely regarded as a new type of soil conditioner that can effectively control soil acidification and alleviate Al toxicity. Hydrochar is identified as a more economical carbon material than pyrochar, but its effect on Al toxicity and the associated mechanism have not been studied. Thus, a two-stage indoor incubation experiment was conducted to investigate the effect of rice-straw hydrochar (HC, application rate: 1/2/3 %) on maize seedling root growth, soil solution Al activity, soil exchangeable Al and pH buffering ability in acidic red soils from two sites. We also used pyrochar (PC, application rate: 3 %) produced from the same rice straw for comparison. Except for HC-1 %, both hydrochar and pyrochar addition significantly stimulated relative root elongation (136.36 % ~ 284.09 %), diminished the cell death ratio (27.96 % ~ 85.56 %) and Al content in root tips (18.80 % ~ 80.11 %) by decreasing the total Al content (44.78 % ~ 76.10 %) and the proportion of Al3+ species (27 % ~ 32 %) in soil solution. Hydrochar did not significantly promote the soil pH buffer capacity (pH-BC) or effective cation exchange capacity (ECEC), while PC-3 % did. The DOC (dissolved organic carbon) content of soil solution was dramatically elevated by 203.9 % ~ 783.2 % after hydrochar addition. Hydrochar mitigates Al activity in soil solution mainly through Al-DOC complexation and adsorption, thus suppressing the Al toxicity of maize roots. Hydrochar may be an economical soil amendment for ameliorating Al toxicity despite its overall alleviation effect on Al toxicity being lower than pyrochar.

Calculation of spatially explicit amounts and intervals of agricultural lime applications at county-level in China.

Liming is a long-established and widely used agricultural practice to ameliorate soil acidity and improve crop production. Sustainable liming strategies for regional applications require information on both lime requirements and liming intervals given land use and soil dependent acidification rates. We developed a method to optimize lime requirements and liming intervals at regional level. Lime requirements were based on soil pH buffering capacity and liming intervals were estimated by ongoing soil acidity production, derived from major cations and anions balances in cropland systems. About 66% of croplands in Qiyang required liming to raise soil pH to 6.5, with a total lime requirement of 2.4 × 105 t CaCO3, with an average rate of 2.4 t ha-1 for paddy soils and 2.6 t ha-1 for upland soils. The remaining 34% were mainly calcareous soils. Nutrient management practices and crop rotations, affecting N transformation and crop removal, were the main drivers controlling the spatial variation in total acid production in non-calcareous soils, on average contributing 73% and 25%, respectively. Under current soil acidification rates, 33% of Qiyang's croplands would need liming within 30 years after raising the soil pH to 6.5. Averaged liming interval was 20 years, and 6.8 t ha-1 would be required to maintain soil pH ranges between 5.5 and 6.5. Areas with high soil acidification risk were mostly located in the southeast of Qiyang.

Striking a balance between N sources: Mitigating soil acidification and accumulation of phosphorous and heavy metals from manure.

Manure amendment has been shown to effectively prevent red soil (Ferralic Cambisol) acidification from chemical nitrogen (N) fertilization. However, information is lacking on how much manure is needed to mitigate acidification and maintain soil productivity while preventing accumulation of other nutrients and heavy metals from long-term inputs. This study determined the effects of various combinations of manure with urea-N on acidification and changes in soil P, K, and heavy metals in a 9-year maize field experiment in southern China. Treatments included chemical N, P and K fertilization only (NPKM0), and NPK plus swine manure, which supplied 20% (NPKM20), 40% (NPKM40), and 60% (NPKM60) of total N at 225 kg N ha-1 year-1. Soil pH, exchangeable acidity, available P and K, and maize yield were determined annually from 2009 to 2018. Soil exchangeable base cations, total and phytoavailable Cr, Pb, As, Ni, Cd, Cu, and Zn were measured in 2018. A significant decrease in soil pH occurred under NPKM0 and NPKM20 from initial 4.93 to 4.46 and 4.71, respectively. Whereas, under NPKM40 and NPKM60 no change or a significant increase in soil pH (to 5.47) occurred, as well as increased exchangeable base cations, and increased yields. Manure application markedly increased soil available P (but not K) to 67.6-182.6 mg kg-1 and significantly increased total Pb, Cu, and Zn and available Cu and Zn in soil. The results indicate sourcing 40% or greater of total N from manure can prevent or reverse acidification of red soil, and provide all P required, however, additional K inputs are required for balanced plant nutrient supply. An integrated approach of increasing N use efficiency, reducing chemical input, and reducing heavy metal concentrations in animal feed are all necessary for sustainable use of manure in soil acidity and nutrient management as well as minimizing environmental risks.

Evaluation of potassium thiosulfate as a nitrification inhibitor to reduce nitrous oxide emissions.

Potassium thiosulfate (KTS, K2S2O3) has been shown to function as a nitrification inhibitor, thus has the potential to reduce nitrous oxide (N2O) emissions and play an important role in effective N management. The objective of this research was to determine the potential effects of KTS on N2O emissions and N transformation processes in comparison with commercial N transformation inhibitors (stabilizers). A laboratory incubation experiment was conducted using urea and ammonium nitrate (UAN) applied at 150mgNkg-1 in a Hanford sandy loam soil (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents). Treatments included three rates of KTS (26, 51, and 102mgS2O32--Skg-1), a urease and nitrification inhibitor (Agrotain® Plus), a nitrification inhibitor (N-Serve® 24), and an untreated control. Nitrous oxide emission, soil pH, and mineral N species were monitored for 35days. Total N2O emissions were reduced significantly by all KTS treatments as a function of KTS rate. At 102mgS2O32--Skg-1, KTS reduced N2O emissions by 48% (0.18% of total inorganic N), which was statistically similar to the N-Serve® 24 treatment (60% reduction) although lower than Agrotain® Plus (78% reduction). The KTS resulted in significantly less unaccounted (total N) loss compared to the commercial inhibitors. If the N2O emissions reductions observed in this laboratory study are validated in the field, using KTS for this purpose can also provide a fertility benefit and may reduce total chemical inputs into agronomic systems. Future research needs to determine the effectiveness of thiosulfate for improving overall nutrient management while reducing N2O emissions under field conditions.

Preparation of Fe-Cu-kaolinite for catalytic wet peroxide oxidation of 4-chlorophenol.

Fe-Cu-kaolinites were prepared by co-precipitation and hydrothermal methods, and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), where 2 wt.% natural kaolinite was dispersed, and the ratios of (Al + Fe + Cu)/clay = 10 mmol/g and Al/(Fe + Cu) = 5/1 were maintained. The effect of different drying methods (vacuum drying, ethanol exchange drying, freeze-drying, microwave drying, normal oven drying) and different Fe/Cu molar ratio (0/2, 0.4/1.6, 0.8/1.2, 1/1, 1.2/0.8, 1.6/0.4, 2/0) was also assessed. Catalytic wet peroxide oxidation (CWPO) reaction of 4-chlorophenol (4-CP) was used to probe the reactivity and activity of the materials prepared. The results showed that Fe and Cu could be successfully intercalated into the interlayer of kaolinite by hydrothermal method, where specific surface area and pore volume increased by 19 times and 7 times, respectively; the intensity of basal space (001) reflection peak was reduced by 80%, and tip width was doubly increased. The catalyst possessed higher reactivity, with 85.5% of 4-CP conversion being observed, whereas only 15.2% of 4-CP was removed over raw kaolinite. High-power microwave drying (720 W) was the best drying method, because it resulted in greater microstructure and thus higher reactivity (85.3% of 4-CP conversion), with lower active metal (Fe or Cu) leaching (3.96 mg L-1). Fe/Cu molar ratio of 0.8-1.0/1.2-1.0 was considered as the optimum ratio in pillaring solution, for maintaining higher catalytic activity (85-90% of 4-CP conversion) and lower metal (Fe or Cu) leaching (7-9.3 mg L-1).

Three-decade long fertilization-induced soil organic carbon sequestration depends on edaphic characteristics in six typical croplands.

Fertilizations affect soil organic carbon (SOC) content but the relative influences of the edaphic and climate factors on SOC storage are rarely studied across wide spatiotemporal scales. This study synthesized long-term datasets of fertilization experiments in six typical Chinese croplands, and calculated annual C input from crops and manure amendments, changes in SOC storage (ΔSOC) and C sequestration efficiency (i.e. the percentage of soil C change per unit of C input, hereafter referred as CSE) in 0-20 cm soil over three decades. Three fertilization treatments include no fertilization (CK), chemical nitrogen, phosphorus and potassium fertilizers (NPK) and combined chemical fertilizers and manure (NPKM). Results showed significant fertilization effects on C input and ΔSOC (NPKM>NPK>CK), and significantly higher CSE in Qiyang at Hunan than Zhengzhou at Henan and Heihe at Heilongjiang. The variance partitioning analysis (VPA) showed more variance of CSE can be explained by edaphic factors (up to 39.7%) than other factors. Furthermore, soil available N content and pH were identified as the major soil properties explaining CSE variance. This study demonstrated key controls of soil fertility factors on SOC sequestration and informs the need to develop strategic soil management plan to promote soil carbon sequestration under long-term intensive fertilization.