Reconsidering “4 per 1000” target in mild salt-affected lands: A case study on exogenous carbon inputs

There is an emerging consensus that exogenous carbon input has the potential to improve soil quality and optimize nitrogen management in salt-affected lands, yet it remains unclear how organic amendments function on crops. Here, we conducted a pot experiment to assess the effect of exogenous carbon input on sweet sorghum and soil properties. Specifically, an unfertilized control (CK), a urea treatment (N) and three combined urea and organic amendment treatments, which represented 2‰ (N-C1), 4‰ (N-C2) and 6‰ (N-C3) increase rate of soil carbon, were included in the experiment. We found exogenous carbon significantly increased nitrogen use rate (NUR) of sweet sorghum in all combined use treatments. It also alleviated the inhibition effect of nitrate accumulation on the growth of sweet sorghum induced by urea, especially at the five-leaf stage, yet still could not offset the negative impact of nitrate on sweet sorghum. Overuse of exogenous carbon led to unintended imbalanced soil C:N, enhanced soluble K + concentration and thereby a decoupling of NUR and biomass accumulation. A 2‰ increase rate of soil carbon by exogenous carbon inputs was optimal for keeping a proper soil C:N mass ratio at about 12:1 for both biomass and NUR of sweet sorghum in coastal salt-affected lands. Our study provides an insight into nutrient management strategies and land carbon carrying capacity for achieving both soil amendment and benefit of crop production in salt-affected lands. • Exogenous carbon inputs alleviated negative effect of nitrate on sweet sorghum. • Nitrogen use rate (NUR) of sweet sorghum was positively increased by exogenous carbon. • Applying exogenous carbon at 2‰ was recommended for improving both biomass and NUR. • Overuse of exogenous carbon led to unintended imbalanced soil C:N, enhanced soluble K + concentration and thereby decoupled biomass and NUR of sweet sorghum.