Hot-water extractable C and N as indicators for 4p1000 goals in a temperate-climate long-term field experiment: A case study from Hungary

•The conversion of crop-land to grassland significantly increased soil C and N fractions.•Labile HWN and HWC fractions were the most sensitive indicators of soil quality changes.•Mineral fertilizer doses producing high crop yields and residues can maintain SOC level.•Only high mineral and organic doses can ensure significant CSR during decades. Soil organic matter (SOM) consists of various labile and stable fractions, which are differently influenced by agricultural activities and land-use change. This study, aimed at investigating the feasibility of achieving 4p1000 goals on conventionally tilled plough-land, was carried out in a 42 years-old long-term field experiment (LTE), in which the effect of three farmyard manure (FYM) doses and the FYM equivalent NPK mineral fertiliser rates can be examined, together with that of mineral fertiliser with and without ploughed-in plant residues. The soil total organic carbon (SOC) and nitrogen (SN) and the labile hot-water soluble C (HWC) and N (HWN) fractions were determined (0–0.3 m) and used as indicators. The parameters of the tilled area were compared with those of a grassland area with similar characteristics. The suitability of these parameters for use as soil quality indicators (SQI) was also examined in terms of soil fertility. The results showed that the most sensitive fraction for the detection of treatment effects were HWC and HWN. Increases in these labile organic fractions were significantly related to the gains of SOC stocks. Based on the close correlation between these factors and both SOC and crop yield it is recommended that they should be used as indicators for the prediction of changes in SOC and in studies on soil fertility or soil quality. The initial C stock of 40.46 Mg ha−1 became 40.27–47.05 Mg ha−1 on the tilled soil after four decades, while on the grassland it rose to 69.31 Mg ha−1. The carbon sequestration rate (CSR) in the various fertilisation systems exhibited the following order: plant residue incorporation >FYM addition > mineral fertilisation (0.147–0.156, 0.101–148 and –0.021–0.065 Mg ha−1 yr−1, respectively), while an outstandingly high value of 0.687 Mg ha−1 yr−1 was recorded for the grassland. Despite the fact that the carbon sequestration potential (CSP) remained high (55.36–62.48%), the achievement of 4 per 1000 aims can only be ensured in treatments involving high rates of organic matter and only in the short term.