Priming of soil organic carbon by malic acid addition is differentially affected by nutrient availability

We investigated the effect of nutrient availability, and soil carbon (C) quality and quantity on the priming effect (PE) resulting from malic acid in a short term (seven day) soil incubation experiment. Two concentrations of 14C labelled malic acid (Low: 100 and High: 1000 mg C kg−1 soil) were added to soils under agricultural management strategies with stubble retention and removal, which have manifested different soil C statuses. Soils were also pre-treated with different soil organic C (SOC) sources to give a wide range of 'native' soil C statuses on which to test the effects of PE by malic acid addition. The PE increased with increasing additions of malic acid, and this was positively influenced by the amount of soil labile C. The nutrient effect on PE was contrasting: the PE induced by high malic acid addition was decreased after nutrient additions whereas the opposite observations were observed in the low malic acid treatments. The positive PE (stimulation of decomposition of SOC) was smaller in low versus high malic acid treatments by a factor of 5.6. The results using pre-treated soils suggested that stable organic C content has a negative effect on the PE induced by malic acid C in soil. We concluded that nutrients increased or inhibited the malic acid induced PE and this was differentially affected by SOC content, thereby potentially impacting C stocks in soil.