Effects of biochar on soil processes, soil functions and crop growth

Biochar is the carbon-rich product obtained when organic material is pyrolyzed, during which bioenergy is produced. When applied to soil, biochar is claimed to have positive effects on soil properties and processes and carbon could be sequestered. For these reasons, biochar production and application to soil is often associated with raising agricultural productivity while mitigating climate change. Especially in (tropical) highly weathered soils, biochar has shown positive effects on soil properties and crop yields, but it is uncertain whether the same positive effects can be obtained in (temperate) more fertile soils. Therefore the overarching aim of this PhD research was to get a better understanding of biochar effects on soil chemical, physical and biological properties, plant growth, and soil greenhouse gas emissions in agricultural northwestern European soils. Lab, pot and field experiments have been conducted to gain insight into biochar effects on plant and soil. 15N tracing lab experiments suggested that in the short term, biochar addition to soil stimulated mineralization of more complex SOC, thereby increasing mineral N availability. However, in the absence of plants this available N was rapidly, biotically immobilized. Furthermore, nitrification rates were increased with biochar addition. In contrast, in the longer term, these effects faded, probably due to the transient effects of biochar labile C fraction and pH. Moreover, lab experiments have shown that biochar can reduce mineral N availability in the short term, likely due to biotic or abiotic N immobilization. It is unknown when and to which extent the immobilized N could become available again. These experiments also show that biochar can increase soil pH, through which several soil processes can be affected, e.g. NH3 volatilization, nitrification and denitrification However, bulk soil pH was not always significantly increased by biochar addition. There was a trend for a higher increase in soil pH after biochar application in low pH soils while at more neutral soil pH, this was not the case as observed in the biochar field trial. It cannot be excluded that elevated pH micro-sites close to biochar particles affect soil processes, despite biochar having no effect on bulk soil pH. In the short term, likely microbial activity and abundance should be altered through biochar, as the 15N tracing experiments showed that biochar affected soil N cycling in the short term. This effect seems to be transient, probably due to a change in biochar properties, as biochar seems to act as an inert substance regarding N cycling in the longer term as biochar seems to act as an inert substance regarding N cycling in the longer term. This was shown by a 15N tracing experiment conducted with soil sampled one year after biochar application. Furthermore, biochar addition to soil did not influence soil microbiological community structure to a large extent in six European North Sea region countries during the first year after biochar application, as only certain bacterial biomarker PLFAs were significantly affected by biochar addition. It was remarkable that fungal biomarker PLFAs were not significantly influenced by biochar addition. Biochar addition to soil reduced N2O and NO emissions compared to the control soil after urea and NO3- fertilizer application, and NO emissions after NH4+ fertilizer application. N2O emissions were more decreased at high compared to low pyrolysis temperatures. Also reduced NO3- availability after biochar addition was observed. We hypothesize that decreased N2O and NO emissions were mediated by multiple interacting phenomena: stimulated NH3 emissions, microbial N immobilization, non-electrostatic sorption of NH4+ and NO3-, and pH effects. Pot trial results showed that biochar can cause short-term reductions in biomass production due to reduced NO3- availability. This effect was biochar feedstock and pyrolysis temperature dependent. Hence biochar addition might in some cases require increased fertilizer N application to avoid crop growth retardation. In the field trial, a complex interaction between soil physical parameters, time after biochar application and time of tillage operations was observed. Effects on bulk density, porosity and soil water retention curves were non-consistent over time, possibly due to interaction with tillage operations. Biochar increased soil water content in 2012, although mostly not significantly. However, in 2013, when soil water content was overall lower compared to during 2012, it was not affected by biochar addition. Under field circumstances, biochar addition to soil did not affect spring barley grain or straw yield, nor N or P uptake during the first two years after biochar application. In the field trial, biochar was applied to soil in autumn, as it was our hypothesis that there could be a negative crop response due to reduced N availability when the biochar would be applied in spring and a crop would be immediately sown. However, biochar did not affect soil mineral N availability, neither immediately after biochar application, nor afterwards. Overall, our results indicate that biochar has mixed effects on soil quality properties in the short term, as effects can be positive, negative, and neutral. The field trial results showed that in medium term, biochar does hardly affect soil properties. Our study shows relatively short-term results, and long-term data are needed to confirm these first results