Trade-offs between greenhouse gas mitigation and nutrient enrichment during tropical composting

Composting provides an opportunity to recycle organic residues into valuable soil amendments; however, amendment selection can create trade-offs between nutrient enrichment and greenhouse gas (GHG) mitigation. Our study evaluated the effects of different organic amendment combinations on GHG emissions and nutrient dynamics during tropical composting. Six cattle manure-based treatments were assessed: cattle manure alone (CM), cattle manure with Tithonia diversifolia (CMT), Tithonia with biochar (CMTB), Tithonia with citric acid (CMTCA), Tithonia with maize stover (CMTM), and Tithonia, maize stover, and citric acid (CMTMCA). We monitored composting for 84 days, measuring CO 2 , CH 4 , and N 2 O emissions alongside changes in temperature, moisture, pH, and carbon, nitrogen, and phosphorus fractions. Greenhouse gas emissions concentrated during the thermophilic phase, the period of greatest emission intensity due to enhanced substrate availability, rapid carbon mineralization, and active microbial transformation. During this phase, cumulative CH 4 emissions ranged from 299 to 1284 g CH 4 –C m −2 , with CMTCA producing 180% higher emissions than CM, while cumulative CO 2 emissions ranged from 10.8 to 14.1 kg CO 2 –C m −2 , with CMTMCA and CMTCA producing 23% and 13% higher emissions than CM, respectively. CMTCA produced the greatest cumulative CH 4 emissions, exceeding CM by 165%. This response reflects the combined influence of Tithonia diversifolia and citric acid addition, where enhanced carbon solubilization increased substrate availability and microbial oxygen demand, potentially promoting transient anaerobic microsites favorable for methane production. Across the composting period, CMTM generated the highest cumulative CO 2 emissions, 18.5% higher than CM, reflecting enhanced carbon turnover from readily degradable substrates in Tithonia diversifolia and maize stover. Biochar amendment (CMTB) increased cumulative N 2 O emissions by 41% compared with CM, suggesting that biochar effects were dependent on interactions with nitrogen-rich Tithonia and compost nitrogen transformation pathways. Conversely, Tithonia-based amendments improved nitrogen enrichment but increased emission risks. Overall, the findings demonstrate that climate-smart composting requires balancing nutrient recovery and GHG mitigation rather than optimising either outcome in isolation. For smallholder farmers in tropical regions, the findings demonstrate that locally available organic amendments can improve compost nutrient characteristics. However, amendment selection should consider potential trade-offs between nutrient enrichment and greenhouse gas emissions.