Biogeochemical cycling of zinc in soil-cassava cropping system in Thailand

Abstract

Interminable plant production with negligent management might lead to zinc (Zn) depletion in soils, which could induce a Zn deficit in staple foods and cause human Zn malnutrition in many of the least-developed countries. Knowledge of the Zn pool and balance in soil-cassava systems is unclear and requires a detailed investigation. Herein, we examined Zn mass inputs (weathering and manure) and output (harvest crop) from 34 locations of cassava-grown soils in Thailand. The results revealed that the median available Zn concentration (0.47 mg kg−1) of the studied soils was below the critical Zn requirement for cassava production. The total Zn stock in the soils varied considerably from 9.0 to 633 kg Zn ha−1, only 1% of which was identified as the available pool, and the rest of the fraction was considered as the reserved pool. Strong linear relationships (R2 > 0.61–0.90) of total Zn concentration with clay, organic matter, and total Fe concentration suggested that phyllosilicate, organic matter, and Fe oxyhydroxides were the primary hosts for Zn in the soils. Belowground biomass (70% of the total biomass) was the most abundant biomass fraction, whereas 88% of the total Zn uptake accumulated in the aboveground biomass. The Zn output from the biomass harvest was 61–642 g Zn ha−1 yr−1, corresponding to only 0.10% of the total soil Zn stock. Most locations (91% of the studied sites) received Zn input solely from mineral weathering (0.014–0.975 g Zn ha−1 yr−1) without Zn fertilizer and manure additions, causing net Zn depletions (–642 to –61 g Zn ha−1 yr−1). A few locations had net Zn accumulations (+123 to + 1,971 g Zn ha−1 yr−1) caused by animal manure additions. Without the manure additions, the total Zn stock was estimated to decline by only 1% of the total current Zn stock within the next 10 years, whereas the manure additions would enhance the total Zn stock by 0.26–7.68%. This study highlighted that the Zn input from weathering is meager for cassava production. Combined Zn fertilizer and organic manure are urgently required to resupply and replenish readily available Zn pools and to promote the total Zn stock for sustaining long-term crop production.