Effects of Different Green Manure Crops Incorporated into Soil on Pepper Cd Uptake and Soil Properties

Objective  This study aimed to investigate the effects of cultivating and incorporating different green manure crops on cadmium (Cd) uptake by subsequent pepper (Capsicum annuum L.) , as well as on soil Cd availability, pH, and other indicators. The goal was to screen the suitable green manure crops, providing new insights for the safe utilization of Cd-contaminated dryland soils.

Methods A pot experiment was conducted with eight treatments: fallow (control), Raphanus sativus L. (oil radish), Brassica napus L. (rape), Lolium perenne L. (ryegrass), Vicia villosa var. glabrescens (hairy vetch), Vicia sativa L. (common vetch), Trigonella foenum-graecum L. (fenugreek), and Phacelia tanacetifolia Benth. (lacy phacelia), with four replicates each. Baseline soil samples were collected before planting to determine pH, organic matter (SOM), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), available potassium (AK), available Cd (ACd), and total Cd (TCd) in soil. After green manure harvest, biomass was measured, and plant nitrogen (N), phosphorus (P) and potassium (K) contents were analyzed. Green manures were incorporated in situ, followed by pepper cultivation. Post-harvest, the Cd contents in pepper were measured, and soil samples were analyzed for pH, OM, TN, HN, AP, AK, ACd and TCd.

Results Compared with fallow land, the incorporation of green manure resulted in significant improvements in soil fertility parameters: The SOM increased by 0.37% - 9.07%, TN contents rose by 2.75% - 11.00%, AN contents elevated by 2.84% - 10.13%, and AP contents improved by 0.85% - 8.78%. Concurrently, soil total Cd contents decreased by 2.44% - 9.71%, while available Cd showed reductions of 2.70% - 20.94%. Except for rapeseed-treated plots where chili pepper Cd content and bioaccumulation coefficient increased by 1.55% and 21.99% respectively, all other green manure treatments exhibited Cd content reductions ranging from 4.51% to 42.25%, with bioaccumulation coefficients decreased by 2.48% - 26.95%. Among these, Vigna radiata var. sublobata demonstrated optimal performance in Cd remediation and soil nutrient enhancement. Chili pepper fruits Cd content was 0.086 mg kg⁻¹ (42.25% reduction vs. fallow), meeting the safety threshold of NY/T 901-2021 (≤ 0.10 mg kg⁻¹), Soil total Cd decreased by 5.39%, available Cd by 20.94%, Soil pH increased by 0.17 units, SOM, total N, AN and AP improved by 7.59%, 8.59%, 9.72% and 2.83% respectively.

Conclusion Vigna radiata var. sublobata proves to be the most effective green manure species for Cd-contaminated soil remediation while ensuring safe chili pepper production.