Carbon dioxide emission, nitrogen mineralisation and spinach dry matter yield in a loamy humic soil amended with lime and poultry manure

While application of lime and poultry manure (PM) increase availability of mineral N in acidic humic soils (> 1.8% organic carbon), these amendments enhance decomposition of soil organic carbon (SOC) increasing carbon dioxide (CO2) emissions. This study investigated the effects of co-application of PM and lime on (i) CO2 emission, (ii) concentration of mineral-N and (iii) spinach dry matter yield and nutrient uptake in a humic soil. Two incubation experiments were set up for 84 days to determine (i) CO2 emission and (ii) mineral-N on soils from the 0–10- and 10–20-cm depths. The treatments were PM (10 t ha−1), lime (12 t ha−1), PM + lime and unamended control. Same treatments (with inclusion of inorganic fertilisers) were applied to a pot trial using soils (0–10-cm depth) that were pre-incubated for 0, 14 and 35 days before planting spinach, and dry matter yield and N uptake were determined. Co-application of PM and lime significantly decreased ammonium-N in soil at both depths but increased nitrate–N and CO2 emission than lime alone. Poultry manure significantly increased ammonium-N and CO2 emission at 0–10-cm depth. For all treatments, cumulative CO2-C was significantly higher at 0–10 cm. Dry matter yield for PM + lime and lime + mineral N was higher than when separately applied. Pre-incubation of soils with lime and PM, separately or in combination, increased dry matter yield. These findings imply that application of PM and lime in humic soils increases mineral-N availability and crop productivity, especially when pre-incubated for 35 days, whilst the increase in CO2 emission could result in the decrease of SOC.