Greenhouse Gas Emissions and Growth of Wheat Cultivated in Soil Amended with Digestate from Biogas Production

Digestate, the product obtained after anaerobic digestion of organic waste for biogas production, is rich in plant nutrients and might be used to fertilize crops. Wheat(Triticum spp. L.) was fertilized with digestate, urea, or left unfertilized and cultivated in the greenhouse for 120 d. Emissions of greenhouse gasses(carbon dioxide(CO_2), methane(CH_4), and nitrous oxide(N_2O)) were monitored and plant growth characteristics were determined at harvest. The digestate was characterized for heavy metals, pathogens, and C and N mineralization potential in an aerobic incubation experiment. No Salmonella spp., Shigella spp., or viable eggs of helminths were detected in the digested pig slurry, but the number of faecal coliforms was as high as 3.6 × 10~4colony-forming units(CFU) g-^(1)dry digestate. The concentrations of heavy metals did not surpass the upper limits established by US Environmental Protection Agency(EPA). After 28 d, 17% of the organic C(436 g kg^(-1)dry digestate) and 8% of the organic N(6.92 g kg^(-1)dry digestate)were mineralized. Emissions of CO_2 and CH_4 were not significantly affected by fertilization in the wheat-cultivated soil, but digestate significantly increased the cumulative N_2O emission by 5 times compared to the urea-amended soil and 63 times compared to the uncultivated unfertilized soil. It could be concluded that digestate was nutrient rich and low in heavy metals and pathogens, and did not affect emissions of CH_4 and CO_2 when applied to a soil cultivated with wheat, but increased emission of N_2O.

Dynamics of ^(14)C-labelled Glucose and NH_4^+ in a Regularly Flooded Extremely Alkaline Saline Soil

Flooding an extremely alkaline(pH 10.6) saline soil of the former Lake Texcoco to reduce salinity will affect the soil carbon(C)and nitrogen(N) dynamics.A laboratory incubation experiment was done to investigate how decreasing soil salt content affected dynamics of C and N in an extremely alkaline saline soil.Sieved soil with electrical conductivity(EC) of 59.2 dS m^(-1) was packed in columns,and then flooded with tap water,drained freely and conditioned aerobically at 50%water holding capacity for a month.This process of flooding-drainage-conditioning was repeated eight times.The original soil and the soil that had undergone one,two,four and eight flooding-drainage-conditioning cycles were amended with 1000 mg glucose-^(14)C kg^(-1) soil and 200 mg NH_4^+-N kg^(-1)soil,and then incubated for 28 d.The CO_2 emissions,soil microbial biomass,and soil ammonium(NE_4^+),nitrite(NO_2^-) and nitrate(NO_3^-) were monitored in the aerobic incubation of 28 d.The soil EC decreased from 59.2 to 1.0 dS m^(_1) after eight floodings,and soil pH decreased from 10.6 to 9.6.Of the added ^(14)C-labelled glucose,only 8%was mineralized in the original soil,while 24%in the soil flooded eight times during the 28-d incubation.The priming effect was on average 278 mg C kg^(-1) soil after the 28-d incubation.Soil microbial biomass C(mean 66 mg C kg^(-1) soil) did not change with flooding times in the unamended soil,and increased 1.4 times in the glucose-NH_4^+-amended soil.Ammonium immobilization and NO_2^- concentration in the aerobically incubated soil decreased with increasing flooding times,while NO_3^- concentration increased.It was found that flooding the Texcoco soil decreased the EC sharply,increased mineralization of glucose,stimulated nitrification,and reduced immobilization of inorganic N,but did not affect soil microbial biomass C.