Impact of organic and conventional farming systems on wheat grain uptake and soil bioavailability of zinc and cadmium

Highlights

• Soil Zn and Cd were promoted by soil organic matter due to higher input and binding.

• Organic farming with compost had more available soil Cd than conventional farming.

• Grain Zn was decoupled from soil Zn.

• Livestock production increased soil and grain Cd indicating contamination.

• In the field, a combination of management and soil properties influenced Zn and Cd.

Abstract

Zinc (Zn) deficiency is a widespread problem in human nutrition and wheat grains are a major source of Zn intake in large parts of the population. It remains unclear to what extent organic and conventional farming practices, differing in organic matter management, influence Zn availability and uptake by wheat grains. Factors leading to an increased Zn uptake may also increase the Cd uptake in wheat grains, which can be harmful for humans. Here, we investigated the effects of different farming practices on Zn and Cd concentrations in wheat grains and their relationships with total and available soil Zn and Cd concentrations, and other soil properties. In northern Switzerland, 28 farms were sampled including 11 organic farms with compost use, 10 organic farms without compost use, and 7 conventional farms without compost use. Soil organic matter was a key factor for soil Zn and especially Cd concentrations across all three farming systems. Total and available soil Cd concentrations as well as soil organic carbon concentration (SOC) were significantly higher on the organic farms with compost use than on the conventional farms. However, only the compost farms with livestock showed significantly higher grain Cd concentrations in comparison to conventional and organic farms without compost use, although a nested effect of cultivar within the system also had an influence. In contrast to Cd, the soil and grain Zn concentrations showed no significant farming system effect although there was a correlation between total soil Zn and SOC when all farms were pooled. Grain Zn was decoupled from soil Zn indicating that under agricultural field conditions the farming systems are a minor factor in increasing grain Zn. Our study suggests that the Zn and Cd soil and grain concentrations were mediated by a combination of on-farm organic matter management, soil properties, cultivar, and livestock production.

Introduction

Increasing zinc (Zn) in edible plant parts is a strategic aim in agricultural systems to combat widespread Zn deficiency in human nutrition (Cakmak, 2008; Cakmak and Kutman, 2017; White and Broadley, 2011). The HarvestPlus program has set a target concentration of 38 mg kg⁻¹ Zn in wheat grains (Bouis and Welch, 2010). In agricultural soils, the availability of soil Zn for uptake by crop plants is limited by its solubility, which is governed primarily by adsorption to mineral surfaces, complexation with organic matter and formation of precipitates (Baird and Cann, 2005; Smolders and Mertens, 2013). Due to its chemical similarity with Zn, the availability of soil cadmium (Cd) is affected in a similar manner by the same soil factors (Hart et al., 2002). However, Cd threatens human health and the environment and, thus, interactions between Zn and Cd availability and uptake should be carefully monitored. In particular, the availability of Zn and Cd in the soil is dependent on soil organic matter (SOM), oxides, clay particles, and pH (Schulin et al., 2009; Smolders and Mertens, 2013). The affinity of Zn and Cd for sorption on soil particles can be >10 times higher on organic matter than on mineral particles (Lair et al., 2007b). As agricultural practises can have a major impact on SOM and soil pH (Bolan et al., 2001; Six et al., 2000), it is important to understand how they influence available Zn and Cd concentrations in agricultural soils. For food safety reasons it is especially important to relate soil Zn and Cd to their uptake by wheat grains to determine how availability translates into the edible parts of crop plants. Previous studies showed that for some cultivars the Zn and Cd accumulation in grains might be more strongly affected by soil properties than others (Gao et al., 2011) whereas the different soil properties of sites had a higher impact than crop cultivars (Oliver et al., 1995; Wångstrand et al., 2007).

Various studies investigated how organic compounds affect Zn and Cd uptake by crop plants. In a hydroponic experiment, the addition of citrate and histidine increased Zn uptake of wheat due to soluble complex formation of Zn with these organic acids, comparing nutrient solutions with the same free Zn concentration (Gramlich et al., 2013). Soil microorganisms can increase soil Zn availability by exuding organic ligands (Altomare and Tringovska, 2011). Mycorrhizal plants were observed to have more Zn available, while Cd toxicity was reduced due to a potential discrimination of the arbuscular mycorrhiza between Zn and Cd (Janoušková et al., 2006). These greenhouse and pot experiments provided important insights, however, the impact of SOM management on Zn and Cd availability remains elusive in real farm environments.

In addition to the complexation by organic ligands and sorption at organic matter surfaces, Zn and Cd can also be sorbed and complexed at mineral surfaces like clay minerals or (hydr-)oxides (Fonseca et al., 2011; Sipos et al., 2008). Organo-mineral associations were shown to provide important interfaces for the binding of heavy metals and can be prominent factors for the binding and mobilisation of soil Zn and Cd (Arias et al., 2005; Leinweber et al., 1995).

The addition of fertilisers or amendments containing organic matter can enrich soils with heavy metals due to metal contents of the applied materials exceeding natural inputs (Facchinelli et al., 2001). Animal feed is supplemented with Zn to improve animal health and livestock productivity and the majority of the supplemented Zn is not retained by the animals and excreted (Bolan et al., 2004; Gubler et al., 2015; Schultheiß et al., 2004). The heavy metal concentrations in manure vary depending on the farming system and the feed and supplements used (Keller et al., 2002; Menzi and Kessler, 1998; Menzi et al., 1999; Möller and Schultheiß, 2015). In German dairy production, on-farm grown fodder introduced 39% of the added Zn and 71% of the Cd into cow sheds, whereas protein-rich external feed accounted for a similar range of Zn and much less Cd (Schultheiß et al., 2004). In addition, the recycling of plant residues and land management was found to influence metal dynamics and plant uptake (Düring et al., 2003; Mench, 1998; Oliver et al., 1993). While many studies have looked at the on-farm flows of heavy metals in livestock production, only a few studies have investigated to what extent heavy metals are retained and mobilised in agricultural soils (Helfenstein et al., 2016).

In this study, we investigated how agricultural cropping and livestock production systems influence Zn and Cd concentrations in wheat grains and how these effects relate to soil Zn and Cd availability and other soil factors. We sampled soils and wheat grains on farms in northern Switzerland categorised in three differing farming systems, with and without livestock production: organic farms with compost use, organic farms without compost use, and conventional farms without compost use. The objectives of the study were (i) to relate total and available soil Zn and Cd concentrations to wheat grain Zn and Cd concentrations and (ii) to assess the role of management practices and soil properties in these relationships.