Objectives This study investigated the effects of within-row organic mulching on soil organic carbon pool and soil enzyme activity to establish an effective soil management practice in walnut (Juglans regia L.) orchards and provide theoretical basis for the promotion of soil nutrients.

Methods A field trail with a random block design was carried out in walnut orchards with non-mulching block as control. The effects of mulching on soil organic carbon pool and soil enzyme activity and their relationship were studied.

Results 1) Organic mulching increased soil organic carbon contents and the effect decreased from surface to deep soil layers. The effect of manure mulching was better than that of wood chip mulching. In 0–20 cm soil layer, the contents of total organic carbon, dissolvable organic carbon, readily oxidizable organic carbon in treatment of manure mulching were 1.66 times, 1.33 times and 6.99 times of those in control, respectively. The soil stable organic carbon content in treatment of wood chip mulching was 1.42 times of that in control. 2) Organic material mulching could increase the carbon pool activity, carbon pool activity index, carbon pool index and carbon pool management index, and the difference among treatments became smaller from surface soil layer to deep soil layers. From top to 80 cm deep of soil, the carbon pool activity and carbon pool activity index in manure mulching treatment were 2.9 to 5.9 times of those in control, and those in wood chips mulching treatment were 1.3 to 2.5 times of those in control. The carbon pool index in manure mulching treatment was 1.1 to 1.7 times of that in control, and in wood chips mulching was 1.1 to 1.5 times of that in control. The carbon pool management index of manure mulching treatment was 3.3 to 9.8 times of that in control, and the same index of wood chips mulching was 1.6 to 2.8 times of that in control. 3) Mulching with organic materials increased the soil enzyme activity. The increase range in manure mulching treatment was 4.2% to 103.0% and that in wood chips mulching was 3.5% to 47.3%. 4) The total organic carbon content was highly and positively correlated with the average carbon pool index, carbon pool activity index and carbon pool management index. A very significant positive correlation was found between soil urease enzyme activity, soil cellulose enzyme activity and total organic carbon content, dissolvable organic carbon content and readily oxidizable organic carbon content.

Conclusions Mulching with manure or wood chips could significantly increase soil carbon sink capacity and improve soil enzyme activity in short term.