Research paper
Short-lived effects of walnut shell biochar on soils and crop yields in a long-term field experiment

https://doi.org/10.1016/j.agee.2016.11.002Get rights and content

Highlights

  • Biochar increased crop productivity in only the second out of four seasons.

  • Increased yield aligned with higher soil K+, Ca2+, and P contributed by biochar.

  • Nitrogen transformations were not effected by biochar under field conditions.

  • Biochar’s effects were short-lived in this high fertility, temperate agroecosystem.

Abstract

Many field studies exploring biochars’ effects on plant productivity and soil quality have been limited to just one or two seasons, particularly in temperate agroecosystems, and have not shown how such impacts change as biochars age in the soil. Therefore, we investigated the lasting effects of a walnut shell (WS) biochar on crop yields and soil nutrient cycling and availability over four years in a field experiment. Long-term plots of a tomato-corn rotation were established in a 2 × 2 factorial design of treatments i) with or without WS biochar amendment and ii) fertilized with mineral fertilizer (MF) or composted poultry manure (CP). Biochar was applied once in 2012 (Year 1) at a rate of 10 t ha−1. Crop yields were measured over four seasons, and soil samples were analyzed for ammonium (NH4+-N) and nitrate (NO3-N) concentrations and for other nutrient parameters, including exchangeable K+, Ca2+, PO4-P, SO4-S, each year. Walnut shell biochar had an effect only in Year 2 when it increased corn yields by ∼8% in both MF and CP fertilizer systems and increased exchangeable K+, PO4-P, and Ca2+ in soil through direct additions of these nutrients. These impacts were not observed until a year after application and faded in subsequent years. Inorganic N pools were not significantly affected by the biochar in any season. The WS biochar has a delayed yet short-lived effect on plant-available nutrient concentrations and crop productivity but does not significantly alter nutrient transformations.

Introduction

The short-term effects of biochar applications on soil nutrient availability and plant productivity have been well studied across a variety of cropping systems and environmental conditions. However, reviews on this topic (e.g. Gul et al., 2015, Lone et al., 2015, Jeffery et al., 2011, Spokas et al., 2012, Huang et al., 2013, Quilliam et al., 2012) continually note the lack of field experiments investigating the effects of biochar over multiple seasons and call for more long-term field trials. One meta-analysis of 371 independent experiments reported that the average study length was 113 days and the longest spanned 3 years (Biederman and Harpole, 2013). The amendment of biochar to a soil is an irreversible decision, and it is therefore critical to evaluate the potential impacts of biochars on crops and soil quality beyond the typical one or two-season experiment cycle. This is especially important given that biochar is not typically intended for annual application, and the logistics of handling and applying it are challenging. Biochar in the soil can persist decades and even centuries beyond non-pyrogenic organic matter (OM) (Wiedner and Glaser, 2015, Gul et al., 2015, Lehmann et al., 2015), but it is also aging over time, altering its interactions with and potential effects on plants and the soil environment. Agricultural management and environmental conditions will affect biochar aging (Verheijen et al., 2010, Jeffery et al., 2011). Therefore, field experiments incorporating realistic farming operations are valuable in determining the changing effects of biochar over time.

The main proposed mechanisms for increased plant productivity involve biochar-induced increases in plant-available nutrients. Changes to nutrient availability can be both direct, such as added potassium (K) weathering from high ash biochars (Major et al., 2010), and indirect, such as changing nutrient solubility by altering soil pH (Gul et al., 2015, Lone et al., 2015). Most biochars have an alkaline pH and contain negatively charged functional groups that bind protons to raise soil solution pH (Gul et al., 2015). Biochars can also increase nutrient retention as these functional groups act as exchange sites for nutrient ions in solution (Major et al., 2010, Gul et al., 2015). This increase in cation exchange capacity (CEC) can reduce leaching and provide a slow-release source for nutrients, particularly in coarse-textured or highly weathered soils containing low activity clay minerals with naturally lower CEC. Effects from direct nutrient additions are likely to quickly diminish as these ions are taken up by plants or leached from the root zone (Major et al., 2010). Indirect mechanisms also vary over time; however, the direction and magnitude of these changes is more unpredictable as they depend on the unique properties of the biochar and the soil to which it is added, as well as climatic conditions of the soil environment. Nutrient retention capacity can increase over time as biochar surfaces become increasingly oxidized (Pignatello et al., 2015), but biochar particles can also become masked by native soil OM, reducing CEC and nutrient retention (Gul et al., 2015). Similarly, the effects of biochar on soil pH tend to decrease over time as charged sites become saturated with H+ and other ions (Gul et al., 2015, Nelissen et al., 2015).

Many studies have focused on the impacts of biochars and other pyrogenic carbon (C) products on nitrogen (N) transformations as a potential explanation for changes in plant productivity. Biochars have the potential to interact with and alter the N cycle at several points due to their chemical properties, influencing C availability and pH, and physical characteristics, such as surface area and aromaticity (DeLuca et al., 2015). Several studies have found that biochars can increase net N immobilization with additions of labile C (Deenik et al., 2010, Rondon et al., 2007, Gundale and DeLuca, 2007), though others have found that this increases microbial activity and mineralization (DeLuca et al., 2015). Biochars may also reduce net mineralization through interactions with organic N substrates, reducing their accessibility to microbes, or by sorbing NH4+ and NH3 on acidic functional groups, removing these products from solution as they are mineralized (Berglund et al., 2004, DeLuca et al., 2015, Asada et al., 2002, Clough and Condron, 2010, Anderson et al., 2014, Taghizadeh-Toosi et al., 2011). Sorption of these substrates may also decrease nitrification rates (Taghizadeh-Toosi et al., 2011), though others have found increased nitrification rates due to stimulation of nitrifying organisms after biochar increases soil pH (DeLuca et al., 2006, Myrold, 2005). A previous mesocosm study, using the same walnut shell (WS) biochar as this current study, found that it doubled net nitrification rates and increased bacterial ammonia oxidizing gene (amoA) abundance (Pereira et al., 2015). Although this information is of value, it is important to test these impacts in realistically managed field systems.

Substantial biochar research has been done in highly weathered, lower fertility soils where reduced CEC can limit nutrient retention, soil acidity controls nutrient solubility, and climatic conditions limit accumulation of OM (Quilliam et al., 2012, Lehmann et al., 2011). However, biochar is still widely promoted in temperate agroecosystems as a means of increasing crop yields and C sequestration. It is therefore important to study the long-term impacts of biochar in more productive agricultural soils where CEC, OM, and extreme pH are less limiting and may buffer the effects of biochar.

This study examines the first four years of a continuing, long-term field experiment in a fine-textured, Alfisol soil located in California’s agricultural region. We investigated the long-term impacts on soil fertility and crop yields of a high temperature (900 °C) WS biochar applied to an intensively managed soil. Soil nutrient cycling dynamics may differ based on the nutrient management system used; thus we studied biochar’s impacts in conjunction with both mineral and organic fertilizers. Our objectives were to determine if WS biochar 1) alters plant-available N pools over the growing season, 2) impacts tomato and corn yields, and 3) has changing impacts as the biochar ages in the soil for several seasons. We hypothesized that 1) soil NH4+-N and NO3-N concentrations would both increase in plots with WS biochar due to increased mineralization and nitrification, 2) corn and tomato yields would be increased in the first two years of the experiment due to increased N availability, 3) the effects of biochar on plant-available N and yield would decrease over time as the exchange sites on the biochar became saturated and there was less interaction of the biochar with the soil solution.

Section snippets

Field experimental design

The long-term experimental biochar plots were established in May 2012 at the Russell Ranch Sustainable Agriculture Facility, a division of the Agricultural Sustainability Institute (2016) at the University of California, Davis (http://asi.ucdavis.edu/programs/rr). This facility oversees farming of plots using the same practices and equipment as commercial growers while controlling their management for experimental measurements.

The 2 × 2-factorial treatment design of this experiment compares

Crop yields

Walnut shell biochar had an effect on crop yields only in Year 2, after it had aged in the field for one year (Fig. 1a and b). In this year, biochar increased corn yields by 8.1% and 7.8% in mineral fertilizer and compost-fertilized plots, respectively (p = 0.0001). This positive effect was not seen in Year 1, the year of biochar application, and it faded in subsequent years as the biochar aged in the field. In contrast, the type of fertilizer used had a significant effect in each season. In

Discussion

This long-term field study offers a realistic view of WS biochar’s integration into farming systems and allows extrapolation across years to determine whether biochar application, an irreversible practice, will have lasting effects. As is the case in all field settings, annual crop productivity is subject to variable factors, such as the timing of water and nutrient availability with demand. Our experiment allowed us to see how the WS biochar’s effects may be changing not only with climatic

Conclusions

Our study explored the long-term effects of WS biochar on crop productivity and soil nutrient availability in conjunction with both organic and mineral fertilizers. The one-time addition of biochar to a high fertility, fine-textured, agricultural soil increased yield only in the second of four seasons. This delayed and short-lived effect can be attributed to significant increases in exchangeable K+, Ca2+, and PO4-P one year after biochar application that did not persist in following years.

Acknowledgements

We are grateful to Jessica Schweiger, Dr. Daniel Bair, Tom Hollemans, and Andrew Margenot for their work in setting up and sampling the experimental plots in 2012, Charlotte Oriol and Phirun Khim for field and lab assistance, Israel Herrera, Luis Loza, and the staff at the Russell Ranch Sustainable Agriculture Facility for their management of the plots, and Russ Lester at Dixon Ridge Farms for donation the walnut shell biochar. This publication was made possible by the United States Department

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