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Three laboratory, continuously stirred tank reactors (CSTRs) co-digesting grass silage and cow manure (forming 30% and 70% of substrate volatile solids (VS), respectively) were operated to evaluate the effects of re-circulating an alkali-treated and untreated solid fraction of the digestate back to the reactors. The CSTRs were operated at an organic loading rate (OLR) of 2 kg VS m(-3) day(-1) and hydraulic retention time (HRT) of 20 days with a semi-continuous mode of feeding. The feasibility of co-digestion with substrate VS containing 30% VS of crop was reinforced, resulting in average specific methane yield of about 180-185 1 CH4 kg(-1) VS. Re-circulation of the solid fraction of digestate back to the reactors in both alkali-treated and untreated forms decreased the methane yield by 11% and 21%, respectively, and resulted in operational problems such as scum formation and accumulation of the reactor materials. Batch studies were conducted to evaluate (i) the methane potentials of the solid fraction of digestate, and whole digestate with alkali treatments ranging from 20-60 g NaOH kg(-1) VS of substrate, and (ii) methane potentials of the accumulated reactor materials as top, middle and bottom layers. The solid fraction of digestate treated with 20 g NaOH kg(-1) VS showed higher specific methane yield (340 l CH4 kg(-1) VS) than the higher range of alkali treatments. The bottom layers of the control reactor and the reactor fed with alkali-treated solids gave a higher specific methane yield (93 and 85 l CH4 kg(-1) VS, respectively), and all three layers of untreated solids gave similar methane potentials.

Soil degradation and C emissions are a threat to sustainable agriculture in many arid and semi-arid areas. For sustainable agriculture, the influence of soil amendments on crop production and soil respiration has been a key focus of research. A three-year field study to assess how soil amendments influence soil properties, soil respiration (Rs), and yield of maize (Zea mays L.) was conducted. Treatments were: no amendment (NA), chemical fertilizer (CF), swine (Sus scrofa L.) manure (SM), maize stover (MS), and swine manure + chemical fertilizer (SC). Soil amendment (CF, SM, MS, and SC) consistently produced greatest grain yield and aboveground biomass, which averaged 38 and 34% greater than NA, respectively. No amendment reduced Rs by an average of 12% compared to amendment treatments. Enhanced grain yield with soil amendment resulted in increased carbon emission efficiency (CEE) with SC>MS>CF>SM>NA. Across years, SC decreased soil bulk density by 13% and increased CEE, soil total C, and soil hydraulic conductivity by 52, 19, and 21%, respectively, compared to NA. These results demonstrate the viability of swine manure + chemical fertilizer at 200 kg N ha−1 as a soil amendment for improved CEE and advancing sustainable maize production in semi-arid rainfed environments.

Purpose Composting is an environmentally sustainable alternative for bioconversion of agricultural residues into a nutrientrich product that can enhance soil fertility/microbial diversity and thereby improve agricultural productivity. The goal of the current study was to evaluate the decomposition pattern of the agro-residues and assess the maturity and phytotoxicity of the composts obtained using physico-chemical, microbial and statistical analyses. The study also attempted to determine a threshold germination index (GI) to serve as a maturity index for the composts by conducting seed germination assays with tomato, chickpea and soybean seeds. Methods Three agricultural residues/straws of Eleusine coracana (finger millet), Sorghum bicolor (sorghum) and Glycine max (soybean) were subjected to aerobic composting for a period of 60 days to study the impact of saw dust on the decomposition pattern and the ultimate compost quality/characteristics. Results The results showed efficient decomposition pattern of the agricultural residues characterized by high temperature profiles (up to 70 °C), high microbial activity, a sharp decrease in C/N ratio of the composting materials, i.e., from an initial 41-61 to final 10-17. Conclusions Statistical evaluation of seed germination assays showed that only the compost obtained from sorghum straw + saw dust was mature and free from any phytotoxicity as all the tested seeds showed higher and statistically significant GIs. It was difficult to attribute a single threshold GI value to indicate maturity of compost and could not further be applied to different types of composts as different seeds responded differently to the same compost.