Forage plant mixture type interacts with soil moisture to affect soil nutrient availability in the short term

Abstract

Agricultural intensification within forage systems has reduced grassland floral diversity by promoting ryegrass (Lolium spp.), damaging soil functionality which underpins critical ecosystem services. Diverse forage mixtures may enhance environmental benefits of pastures by decreasing nutrient leaching, increasing soil carbon storage, and with legume inclusion, reduce nitrogen fertilizer input. This UK study reports on how species-rich forage mixtures affect soil carbon, phosphorus, and nitrogen at dry, medium and wet soil moisture sites, compared to ryegrass monoculture. Increasing forage mixture diversity (from 1 to 17 species) affected soil carbon at the dry site. No effect of forage mixture on soil phosphorus was found, while forage mixture and site did interact to affect soil nitrate/nitrite availability. Results suggest that forage mixtures could be used to improve soil function, but longer-term studies are needed to conclusively demonstrate environmental and production benefits of high-diversity forages.

Introduction

The main function of agricultural soils is to support food production, but they also provide many other ecosystem services (Milne et al., 2015). UK land use is dominated by grassland systems whose primary purpose is to provide feed for ruminants. Agricultural intensification has led to the dominance of ryegrass (Lolium spp.), which is considered the most profitable species in these systems (Hopkins & Wilkins, 2006). This simplification, however, comes at a cost of the reduction of many ecosystems functions (Cong et al., 2014). Diverse forage mixtures may alleviate some issues as they contain species with varying phenology, root depth, and biomass when compared to lower diversity grassland mixtures (Skinner & Dell, 2016). Soil nutrient cycling and retention can be greatly influenced by species diversity, even in the short term of just two years (Steinbeiss et al., 2008). Along with increasing human population and related demand for food, producers will also face climate change, with increased probability of extreme weather events such as droughts or flooding (Hopkins & Del Prado, 2007). Given their extent, forage systems must be able to maintain productivity, but should also contribute to climate change mitigation and soil sustainability. The objective of this paper is to evaluate potential benefits of forage mixtures other than ryegrass for soil health indicators along a water availability gradient. We hypothesized that forage mixture and prevailing soil moisture conditions interact to affect (H1) soil carbon content, (H2) nitrate/nitrite availability, (H3) soil ammonium content, and finally (H4) plant availability of soil phosphorus.

Materials and Methods

The experimental site is located at the University of Reading Farm in Sonning, Berkshire, UK (51^o28’22.4”N 0^o54’15.3”W). Three locations within the farm site were chosen for their varying soil moisture regime: dry (2%), medium (7%), and wet (14% soil moisture as of June 2018). Each of the sites was treated homogenously at least for 10 years prior to the experiment. At each location, four replicates of 4 forage mixtures in a 4x4 Latin square design were sown in September 2016. Each of the mixture plots was 4.2 x 5 m in size, the plots were a single species perennial ryegrass (R) and three forage mixtures: Smart Grass (6 species), Biomix (12 species), and Herbal (17 species, Table 1). Ten 2 cm diameter x 15 cm depth soil core samples were taken from each plot in June 2018, nearly two years after establishment and analysed for carbon, nitrogen and phosphorus content. Effects of forage mixture and site were evaluated using R studio (R Core Team, 2018) with plotrix package (Lemon, 2006). A detailed description of the field setup and analytical methodology is available at dx.doi.org/10.17504/protocols.io.bfrajm2e.

Table 1. Forage mixture species selection list (R: Ryegrass; SG: Smart Grass; B: Biomix; H: Herbal)

Results

A long-term effect of site on soil carbon concentration was found (P < 0.001; Figure 1), but there was no effect of forage mixture (reject H1). At the dry site, soil carbon content was greater in the R treatment compared to the Smartgrass and Herbal mixtures (P < 0.01 for both). There was an interactive effect of forage mixture and site on soil nitrogen from nitrate/nitrite (P < 0.05; Figure 2). More nitrate/nitrite accumulated in the soil under Smartgrass and Herbal mixtures as soil moisture increased. There were significant differences in soil ammonium availability between sites (P < 0.001; Figure 3). But no interaction between species and site on soil ammonium concentration (reject H3). Soil phosphorus concentrations differed significantly between sites (P < 0.001), the dry site had the highest concentration of soil phosphorus, while the medium soil moisture site had the lowest. No interactive effect of forage mixture and site on soil phosphorus concentration was found (reject H4) (see Table 2 for data).

Fig. 1. Boxplot of log soil carbon (C) content in dry, medium soil moisture and wet sites as affected by plant species mixtures: R – Ryegrass (1 species); SG – Smart Grass (6 species); B – Biomix (12 species); and H – Herbal (17 species). Boxes show median, middle 50% of data and upper and lower quartile data range. Letters denote significant difference between treatments.

Fig. 2. Boxplot of soil nitrate/nitrite (NO₂.N/NO₃.N) content in the dry, medium soil moisture and wet sites as affected by plant species mixture: SG – Smart Grass (6 species); B – Biomix (12 species); and H – Herbal (17 species). Ryegrass data were not considered due to nitrogen application in that treatment. Boxes show median, middle 50% of data and upper and lower quartile data range. Letters denote significant difference between treatments.

Fig. 3. Boxplot of log nitrogen from ammonium (NH₃.N) in the dry, medium soil moisture and wet sites as affected by plant species mixture: SG – Smart Grass (6 species); B – Biomix (12 species); H – Herbal (17 species). Ryegrass data were not considered due to nitrogen application in that treatment. Boxes show median, middle 50% of data and upper and lower quartile data range. Letters denote significant difference between treatments.

Table 2. Ammonium (NH₃), nitrite (NO₂), nitrate (NO₃) and phosphorus (P) concentration (mg/kg,) and carbon (C, %) in oven dried soil (mean ± standard error). Forage mixtures used in this experiment were R – Ryegrass (1 species); SG – Smart Grass (6 species); B – Biomix (12 species); H – Herbal (17 species).

Discussion

The lack of any significant effect of species mixture on soil carbon content found in the present study conflicts with previous work showing that increasing species diversity positively affects soil carbon accumulation (Skinner & Dell, 2016), possibly due to the relatively short-term nature of the current study. Skinner and Dell (2016) measured soil carbon around a decade after experimental treatment establishment. However, increased carbon accumulation in the top 5 cm soil profile can occur after just 2 years; and in the top 20 cm within 4 years (Steinbeiss et al., 2008). Had the current samples been split into three 5 cm sampling depths, carbon accumulation may have been observed in the surface layer.

The wet site had a significantly higher soil carbon content than the medium or dry sites (Figure 1), likely due to higher water availability stimulating primary productivity and thus carbon deposition. With the predicted increase in temperature in parts of the UK, the dry site could give us an insight into future abilities of pastures to store soil carbon. In this instance, the 12 species forage mixture accumulated significantly more soil carbon than the 6 species mixture (Figure 1).

Forage mixture and soil moisture availability showed an interactive effect on soil nitrate/nitrite concentrations (Figure 2). More soil nitrate/nitrite was found under the Biomix and Herbal mixtures compared to the Smartgrass mixture at the wet site. This could be attributed to the higher proportion of sown legumes in these two mixtures (Biomix 220 g/kg; Herbal 370 g/kg) compared to the Smartgrass mixture (80 g/kg), confirming earlier studies showing effects of legume inclusion on soil nutrient concentrations (Cong et al., 2014).

Forage mixture had no significant effect on the concentration of nitrate/nitrite at the medium site. The contrast between the medium and the other two sites alludes to the fact that mixtures may require more time to establish full functionality and to demonstrate any significant changes in soil chemistry from the addition of diverse forage species such as legumes under non-stressed environments (Cong et al., 2014).

Soil ammonium concentration was lower at the wet and medium sites compared to the dry site (Figure 3). Biological reasons for the low ammonium concentration at the medium and wet sites may include: (i) water not a limiting factor for plant growth, stimulating ammonium uptake; or (ii) denitrification and/or volatilization being higher in the wetter soils so more nitrogen is lost to the atmosphere than at the dry site.

There was no significant effect of plant species mixture on soil phosphorus availability. The dry site had higher soil phosphorus availability than the medium and wet sites. Years of arable cropping and fertilizer application at the dry site may explain such high phosphorus concentrations compared to the other two sites, known as the legacy effect (Van der Putten et al., 2013).

Conclusion

The current short-term findings suggest that there is a significant interactive effect of forage mixture and inherent site soil moisture content on soil nitrate/nitrite concentration. Timing of sampling, both in terms of elapsed time since sowing of mixtures and seasonal sample collection, may affect the observations. Farmer adoption of forage mixtures may be an important measure to realise environmental improvements such as increased carbon sequestration or decreased phosphorus and nitrogen leaching. Further studies are therefore required to define which forage mixtures encourage longer-term pasture sustainability, with environmental benefits both above and below ground, in addition to economic and social benefits.