Cattle manure application for 12 and 17 years enhanced depth distribution of soil organic carbon and X-ray computed tomography-derived pore characteristics.

Long-term fertilizer application in row crops may influence soil pore characteristics, thereby impacting soil aggregation and structure. Therefore, understanding the influences on soil pore characteristics is useful for adopting suitable conservation practices. However, the impact of cattle manure and inorganic fertilizer application at varied rates on soil pore characteristics in the soil profile at a microscale level remains limited. This study quantifies the impacts of manure and inorganic fertilizer amendments under a corn (Zea mays L.)-soybean (Glycine max L.)-spring wheat (Triticum aestivum) rotation system on soil pore characteristics using the X-ray computed tomography (XCT). Treatments included: low manure (LM; 4.4 and 3.3 Mg ha-1), medium manure (MM; 27.4 and 18.7 Mg ha-1), high manure (HM; 54.8 and 37.4 Mg ha-1), medium fertilizer (MF; 136 kg N ha-1, 49 kg P2O5 ha-1, and 91.5 kg K2O ha-1), high fertilizer (HF; 204 kg N ha-1, 73.5 kg P2O5 ha-1, and 137.3 kg K2O ha-1), and control (CK), respectively, at Brookings (initiated in 2008) and Beresford (2003) in South Dakota. Four intact soil cores were collected from each treatment at 0-10, 10-20, 20-30, and 30-40 cm depths. Results showed that the HM treatment increased the SOC by 8-68% compared to the CK and MF at 0-20 cm at the study sites. Both HM and MM treatments increased the macroporosity and mesoporosity in 0-20 cm soil depths at both study sites. Treatment did not always improve soil pore characteristics below 20 cm soil depth. Additionally, a positive correlation was observed between the XCT-derived macroporosity, total number of macropores, and SOC for all the treatments. Therefore, this study encourages the adoption of the XCT technique in quantifying soil pore characteristics and suggests that long-term medium manure application enhances soil structure as compared to an equivalent inorganic fertilizer application.

Co-application of biochar and nitrogen fertilizer reduced nitrogen losses from soil.

Combined application of biochar and nitrogen (N) fertilizer has the potential to reduce N losses from soil. However, the effectiveness of biochar amendment on N management can vary with biochar types with different physical and chemical properties. This study aimed to assess the effect of two types of hardwood biochar with different ash contents and cation exchange capacity (CEC) on soil N mineralization and nitrous oxide (N2O) production when applied alone and in combination with N fertilizer. Soil samples collected from a temperate pasture system were amended with two types of biochar (B1 and B2), urea, and urea plus biochar, and incubated for 60 days along with soil control (without biochar or urea addition). Soil nitrate N, ammonium N, ammonia-oxidizing bacteria amoA gene transcripts, and N2O production were measured during the experiment. Compared to control, addition of B1 (higher CEC and lower ash content) alone decreased nitrate N concentration by 21% to 45% during the incubation period while the addition of B2 (lower CEC and higher ash content) alone increased the nitrate N concentration during the first 10 days. Biochar B1 also reduced the abundance of amoA transcripts by 71% after 60 days. Compared to B1 + urea, B2 + urea resulted in a significantly greater initial increase in soil ammonium and nitrate N concentrations. However, B2 + urea had a significantly lower 60-day cumulative N2O emission compared to B1 + urea. Overall, when applied with urea, the biochar with higher CEC reduced ammonification and nitrification rates, while biochar with higher ash content reduced N N2O production. Our study demonstrated that biochar has the potential to enhance N retention in soil and reduce N2O emission when it is applied with urea, but the specific effects of the added biochar depend on its physical and chemical properties.

Spatiotemporal patterns and drivers of methane uptake across a climate transect in Inner Mongolia Steppe.

Steppe soils are important biological sinks for atmospheric methane (CH4), but the strength of CH4 uptake remains uncertain due to large spatiotemporal variation and the lack of in situ measurements at regional scale. Here, we report the seasonal and spatial patterns of CH4 uptake across a 1200 km transect in arid and semi-arid steppe ecosystems in Inner Mongolia, ranging from meadow steppe in the east plain to typical and desert steppes on the west plateau. In general, seasonal patterns of CH4 uptake were site specific, with unimodal seasonal curves in meadow and typical steppes and a decreasing seasonal trend in desert steppe. Soil moisture was the dominant factor explaining the seasonal patterns of CH4 uptake, and CH4 uptake rate decreased with an increase in soil moisture. Across the transect, CH4 uptake showed a skewed unimodal spatial pattern, with the peak rate observed in the typical steppe sites and with generally higher uptake rates in the west plateau than in the east plain. Soil moisture, together with soil temperature, soil total carbon, and aboveground plant biomass, were the main drivers of the regional patterns of CH4 uptake rate. These findings are important for model development to more precisely estimate the soil CH4 sink capacity in arid and semi-arid regions.

Effects of field-grown transgenic switchgrass carbon inputs on soil organic carbon cycling.

Genetic engineering has been used to decrease the lignin content and to change the lignin composition of switchgrass (Panicum virgatum L.) to decrease cell wall recalcitrance to enable more efficient cellulosic biofuel production. Previous greenhouse and field studies showed that downregulation of the gene encoding switchgrass caffeic acid O-methyltransferase (COMT) and overexpression of the switchgrass PvMYB4 (MYB4) gene effectively improved ethanol yield. To understand potential environmental impacts of cultivating these transgenic bioenergy crops in the field, we quantified the effects of field cultivation of transgenic switchgrass on soil organic carbon (SOC) dynamics. Total and active SOC as well as soil respiration were measured in soils grown with two COMT-downregulated transgenic lines (COMT2 and COMT3), three MYB4-overexpressed transgenic lines (L1, L6, and L8), and their corresponding non-transgenic controls. No differences in total SOC, dissolved organic carbon (DOC), and permanganate oxidizable carbon (POXC) were detected between transgenic and non-transgenic treatments for both COMT (10.4-11.1 g kg-1 for SOC, 60.0-64.8 mg kg-1 for DOC, and 299-384 mg kg-1 for POXC) and MYB4 lines (6.89-8.21 g kg-1 for SOC, 56.0-61.1 mg kg-1 for DOC, and 177-199 mg kg-1 for POXC). Soil CO2-carbon (CO2-C) production from the COMT2 transgenic line was not significantly different from its non-transgenic control. In contrast, the COMT3 transgenic line had greater soil CO2-C production than its non-transgenic control (210 vs. 165 µg g-1) after 72 days of laboratory incubation. Combining the improvement in ethanol yield and biomass production reported in previous studies with negligible change in SOC and soil respiration, COMT2 could be a better biofuel feedstock than COMT3 for environmental conservation and cost-effective biofuel production. On the other hand, MYB4 transgenic line L8 produced more biomass and total ethanol per hectare while it released more CO2-C than the control (253 vs. 207 µg g-1). Long-term in situ monitoring of transgenic switchgrass systems using a suite of soil and environmental variables is needed to determine the sustainability of growing genetically modified bioenergy crops.