Elsevier

Biomass and Bioenergy

Volume 70, November 2014, Pages 429-439
Biomass and Bioenergy

Phosphorus availability and soil microbial activity in a 3 year field experiment amended with digested dairy slurry

https://doi.org/10.1016/j.biombioe.2014.08.004Get rights and content

Highlights

  • The fertilizer values of a biogas digestate and an undigested substrate were compared.

  • Attention was also given to the effects of the substrates on soil microbial activity.

  • Both substrates had positive effects on maize P nutrition and P availability in soil.

  • Activities of soil enzymes were relatively low after application of the digestates.

  • The CO2 efflux from the soil surface did not depend on the type of substrate applied.

Abstract

The application of biogas residues to agricultural fields is important for nutrient cycling. A 3 year field experiment was conducted to assess the phosphorus (P) fertilizer value of digestate from biogas production, taking into account soil microbial activity. The input substrate (inputS) and digested substrate (digestS) from a biogas plant using dairy slurry, maize silage and wheat corn, were applied at a rate of 30 m³ ha−1 annually. For control, mineral N and K, but no P, were applied in equal amounts with the biogas substrates. Maize was cultivated every year, and the biomass yield and P and N uptake were determined. Soil samples were collected on different sampling dates, and the P contents, pH, organic matter contents and enzyme activity were analyzed. The CO2 efflux was measured biweekly using a portable soil respiration chamber (EGM 4). After 3 years, the P and N uptake increased by 25% in the digestS treatment compared with that of the control but did not differ from that of the inputS treatment. The plant-available P contents were also higher in the inputS- and digestS-amended soil. The fertilizer application did not influence the organic matter content but did influence the enzyme activity in soil. Averaging of all the sampling dates in 2010 and 2011, the activities of dehydrogenase and alkaline phosphatase were 50% lower in the soils that were amended with digestS compared with inputS. However, the CO2 efflux from the soil surface was the same for the inputS and digestS treatments. Our results indicate that the anaerobic digestion of substrates does not affect the plant P uptake but the performance of soil microorganisms.

Introduction

The residues from biogas production are increasingly being used as fertilizers in crop production. It is estimated that approximately 65 hm3 biogas residues (digestate) are produced annually in Germany [1]. Most on-farm biogas installations use animal slurries with crop and fodder residues or dedicated energy crops as input substrates [2]. The anaerobic digestion process alters the composition of the input substrate, potentially affecting plant nutrition and soil properties when the digestate is applied to the field. The total nutrient contents generally remain constant during anaerobic digestion, but the organic matter contents and the C/N ratio decrease, and the mineral N (NH4–N) contents and substrate pH increase [1], [3]. In contrast, the P solubility might decrease after anaerobic digestion due to the formation of struvite (MgNH4PO4 * 6H2O) and poorly soluble hydroxylapatite compounds (Ca5(PO4)3OH) [4], [5], [6].

Many studies have investigated the N fertilizer value of the digestate. In various incubation and pot experiments the digested slurry increased the plant-available N contents in the soil, the crop N uptake and the crop yields compared with the undigested slurry [7], [8], [9], [10]. Under field conditions, the positive effect of the digested slurry on the plant N uptake often was less pronounced or occurred only during the early growth stage [11], [12], [13], [14]. So far, less is known about the P-fertilizer effect of biogas residues. Recent studies have focused more on the effect of digestate on the soil organic matter pool and soil biology. During anaerobic digestion the easily degradable organic compounds become mineralized [15], [16], [17], and it can be expected that less but more stable organic matter is returned to field with the digestate. The mineralization of organic carbon when applied as digestate was often found to be lower than that from undigested materials [7], [16], [18]. For example, the cumulative CO2 emissions were 2.5 times lower in the soils that were amended with digested maize straw than in soils that were amended with raw maize straw over a 21 day incubation period [19]. Johansen et al. [20] found that after a 9 day incubation period, the total CO2 emissions from co-digested cattle slurry were 25% less then when raw cattle slurry was applied. So far, no differences between the undigested and (co-) digested slurry treatments have been found regarding the soil organic carbon content [9], [20], [21]. However, several laboratory and pot experiments have reported a decrease in the biomass of endogeic earthworm species, microbial biomass and dehydrogenase, β-glucosidase, cellobiohydrolase and xylanase activities in soils that were fertilized with digestate compared with soils that were fertilized with undigested slurry [9], [19], [22], indicating that the organic matter in the digestate can hardly be used as carbon and energy source by soil (micro-) organisms.

The P availability in the soil is controlled by the activity of the soil microorganisms. Digestate application may thus affect the P availability in the soil and plant P nutrition directly by adding inorganic and organic P compounds but also indirectly by affecting soil microorganisms due to the supply of nutrients organic matter. Soil microorganisms produce and excrete protons, organic anions and enzymes, thereby mobilizing P from organic and inorganic compounds and increasing the plant-available P pool in the soil [23]. In addition, the bioavailable P pool in the soil could be depleted due to the incorporation of P into the microbial biomass. Because the turnover of microbial biomass in the soil is rapid, the microbial-bound P can still be regarded as labile, readily available and is protected against fixation and leaching [23], [24].

Studying the P-fertilizer effect of biogas residues is of particular importance because natural P resources will be exhausted in the short to medium term [25], [26] and the recycling of biogas residues in agriculture is an important factor to realize nutrient cycling and to save mineral fertilizers. Therefore, this study investigated the effect of anaerobic digestion on the P fertilizer value of slurries under field conditions, taking into account the impacts on soil microbial activity. We hypothesized that changes in the composition of organic fertilizers due to the biogas process affect the P and C turnover in the soil when supplied with digestate, which, in turn, affects the plant yield and plant P nutrition.

Section snippets

Study site

An on-farm experiment was established in September 2008 in close cooperation with a local dairy farm. The experimental area is located in Mecklenburg-West Pomerania at 53°42′ northern latitude and 12°53′ western longitude. The climate of the region is characterized by an average annual precipitation of 550 mm and an average annual air temperature of 8.1 °C (German National Meteorological Service, DWD [27]). The experimental area has not received any organic fertilizer since 1995. The soil type

Effect of inputS and digestS on maize growth and nutrient uptake

In the 3 year field experiment, we analyzed the effect of anaerobic digestion on the fertilizer value of slurries, focusing on the element P. The results indicate a clear effect of the fertilizer type and the experimental year on the maize yields and P and N uptake. In general, these values were higher in the inputS and digestS treatments than in the control treatments. However, significant results were only obtained in 2011, after 3 years of continuous substrate application. Here, the P and N

Plant nutrient uptake and P solubility in the soil

The maize P and N uptake was higher under the inputS and digestS treatments than under the control treatments. However, significant differences between the fertilizer treatments were found only in the third experimental year. Commonly, under practical conditions, it takes several years for new management practices to affect the yield parameters [33]. P release from the soil into the soil solution most likely contributed to the crop P nutrition, resulting in relatively high yields in the control

Conclusion

Due to anaerobic digestion, the composition and nutrient availability of the digestate changed compared to that of the input substrate. In our 3 year field trial, this change did neither affect the N and P uptake of maize nor the P availability in the soil. Therefore, residues from the anaerobic digestion of slurries represent a valuable and readily available P source for crops. However, the lower activities of soil microorganisms and P cycle-related enzymes indicated a lower C and P turnover

Acknowledgments

We thank the Landesgraduiertenförderung Mecklenburg Western-Pommerania for providing the grant and supporting this research.

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