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Promising agronomic effects of biochar addition have been found in a wide range of latitude with low-fertile soils (Biederman and Harpole, 2013), due to improvements of soil biological, physical or chemical properties (Cornelissen et al., 2013, Glaser et al., 2002, Lehmann and Rondon, 2006, Yamato et al., 2006). Biological effects may include enhanced activities of mycorrhizal fungi, ameliorating nutrient uptake by plants (Atkinson et al., 2010) and increased colonization rates of arbuscular mycorrhizal fungi, which for maize plant roots have been shown to increase significantly by 26% for biochar amended soils applied at a rate of 10 l m− 2 (around 20 t ha− 1) Yamato et al., 2006).
With regard to soil physical properties, biochar addition improved soil water holding capacity (WHC) and plant available water (PAW) in both loamy and sandy loam soils (Bruun et al., 2014, Dugan et al., 2010, Martinsen et al., 2014). WHC was increased by 11% upon biochar amendment (9 t ha− 1) in a silty loam agricultural soil, Southern Finland (Karhu et al., 2011). Increased PAW upon biochar addition can be explained by improved porous structure (both microporosity and mesoporosity) and soil aggregation (Herath et al., 2013, Obia et al., 2016). Although it is apparent that biochar can improve soil moisture, there is a knowledge gap regarding to what extent this effect can explain the positive effect of biochar on crop growth.
In addition to soil physical properties, soil chemical properties can also be improved significantly by the addition of biochar. Besides increasing soil pH (higher Ca/Al ratios and higher PO4− 3 availability) and base saturation (BS) (Glaser et al., 2002, Martinsen et al., 2015) the addition of biochar increases nutrient retention capacity and soil CEC (Chan et al., 2008, Liang et al., 2006) and thus reduces nutrient leaching (Hale et al., 2013, Laird et al., 2010, Martinsen et al., 2014, Steiner et al., 2007). Low pH is commonly associated with increased Al-concentrations in soil solution, which is highly toxic to plant roots (Gruba and Mulder, 2008). The Al concentration can be reduced drastically by addition of biochar that acts as a liming agent in most of the degraded soils (Glaser et al., 2002, Major et al., 2010, Martinsen et al., 2015, Van Zwieten et al., 2010, Yamato et al., 2006). When 20 t ha− 1 biochar was applied on highly weathered tropical soils, soil pH increased from 3.9 to 5.1, thereby reducing exchangeable Al3 + from 2.67 to 0.12 cmolc kg− 1 and exchangeable H+ from 0.26 to 0.12 cmolc kg− 1 whereupon maize yield almost doubled (10 t ha− 1) compared with control soils (5 t ha− 1) (Yamato et al., 2006). Similar trends were observed for soil CEC, base saturation and exchangeable K upon biochar addition.
Biochar addition can have a strong influence on in-situ soil nutrient availability, emphasizing its role in soil nutrient adsorption and plant availability. Biochar produced from peanut hull at 600 °C showed reduced leaching of NH4+-N, NO3-N and PO4-P (35%, 34% and 21%, respectively) under ex-situ conditions (Yao et al., 2012). The main mechanism may be the absorption of NO3-N in biochar nano-pores (Kammann et al., 2015). PO4-P is tightly bound in highly weathered tropical soils that are often rich in Fe and Al oxides (Hale et al., 2013). Under such conditions, biochar addition increases soil pH which makes PO4-P more bio-available in soil solution (Asai et al., 2009, Hale et al., 2013). For many biochars, an increase in soil K availability may be due to high content of K in biochar ash and reduced K leaching upon biochar addition (Laird et al., 2010, Martinsen et al., 2014). Adding biochar during composting results in organic coatings being formed in the biochar pores (Hagemann et al., 2017), which retains and facilitates slow release of most important plant nutrients (Joseph et al., 2018). Similar to the effect of biochar on soil moisture, it is often unclear to what extent biochar's effect on nutrient retention explains its positive agronomic effects.
We studied the role of biochar in improving soil fertility for maize production. The effects of biochar on the alleviation of three potential physical-chemical soil limitations for maize growth were investigated, i.e. water stress, nutrient stress and acid stress. Experiments involved soils with two dosages of biochar (0.5% and 2% w:w), as well as ones without biochar, in combination with four different dosages of NPK fertilizer, water and lime. Biochar was produced from the invasive shrubby weed Eupatorium adenophorum using flame curtain kilns. This is the first study to alleviate one by one the water stress, nutrient stress and acid stress in order to investigate the mechanisms of biochar effects on soil fertility.
Biochar addition increased soil moisture, potassium (K) and plant available phosphorous (P-AL), which all showed significant positive relationship (p < 0.001) with above ground biomass of maize. However, biochar was much more effective at abundant soil watering (+ 311% biomass) than at water-starved conditions (+ 67% biomass), indicating that biochar did increase soil moisture, but that this was not the main reason for the positive biomass growth effects. Biochar addition did have a stronger effect under nutrient-stressed conditions (+ 363%) than under abundant nutrient application (+ 132%). Biochar amendment increased soil pH, but liming and pH had no effect on maize dry biomass, so acidity stress alleviation was not the mechanism of biochar effects on soil fertility.
In conclusion, the alleviation of nutrient stress was the probably the main factor contributing to the increased maize biomass production upon biochar addition to this moderately acidic Inceptisol.

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