**4.2 Biogas digestate mixed with crop residue to mitigate nitrate leaching risk**

Several management strategies have been proposed to mitigate nitrate leaching: (i) limiting N application rates, (ii) synchronizing N supply to plant demand, (iii) adopting cover crop techniques, (iv) using nitrification inhibitors, and (v) applying a C source such as wheat or rice straw [85]. Incorporating digestate with straw residue from harvested crops is a promising practice to retain NO3 <sup>−</sup> in the soil. Crop residue with a low C/N ratio degrades fast [86, 87], which increases the soil

microbial biomass [88] and stimulates net N mineralization [87, 89]. In contrast, crop residue with a high C/N ratio stimulates net N immobilization, leading to a lower risk of NO3 <sup>−</sup> leaching [90]. Previous studies have indicated that applying organic manure [91] or mineral N fertilizer [92] with straw (high C/N ratio) into cultivated soils reduced the accumulation of NO3 <sup>−</sup> in the soil, since soil microbes use labile C contained in straw as an energy and carbon source with rapid microbial N immobilization [93, 94], thus decreasing NO3 <sup>−</sup> leaching [95].

Wang [82] showed that NO3 <sup>−</sup> concentration was the lowest in the treatment of biogas digestate mixed with a high amount of rice straw to adjust the C/N ratio from 12 to 30 (Mix2). The NO3 <sup>−</sup> concentration in soil was much lower in Mix2 for a 90-day incubation period than in the other treatments, such as only biogas digestate and chemical fertilizer, indicating that most of the N added to Mix2 was microbially immobilized. Other studies also indicated that application of straw induced net N immobilization during the initial stages and released N at a later stage and the timing is largely dependent on climatic and soil factors including soil fertility [96–98]. It has been reported that application of crop residues reduces N losses and causes greater N retention in soil [99]. Yang [94] showed from a 5-year field experiment that straw application reduced soil NO3 <sup>−</sup> leaching losses by 13% compared with the control treatment.

It is a matter of concern when N transformation process changes from immobilization to mineralization. In Kikugawa soil (pH = 7.0), the markedly low NO3 <sup>−</sup> in Mix2 started to increase from day 35, indicating the net re-mineralization of the once immobilized N and soil organic N from day 35. In contrast, in Fuchu soil (pH = 5.7), NO3 <sup>−</sup> started to increase only after day 60, indicating that microbial immobilization consistently dominated the nitrogen cycling process for the first 60 days. The period of N retention and N supply processes differ among soils [100]. Zhao [101] reported that N retention was much longer in a soil with lower pH (5.3) than in a soil with neutral pH (7.6). Soil fertility may also be involved in the change from N immobilization to N mineralization, since Pan [95] reported that N mineralization starts earlier in a fertile soil after the occurrence of N immobilization. Kikugawa soil (total C: 73.2 g kg<sup>−</sup><sup>1</sup> soil) showed higher fertility than Fuchu soil (total C: 35 g C kg<sup>−</sup><sup>1</sup> soil), and thus the earlier change from N immobilization to N mineralization occurred in fertile Kikugawa soil.

### **4.3 Effect of biogas digestate application on root-knot nematode**

Root-knot nematodes (*Meloidogyne* spp.) are the most economically damaging group of plant-parasitic nematodes (PPNs) worldwide [102–104]. The genus *Meloidogyne* is composed of approximately 100 species and parasitizes thousands of plant species [105, 106]. This parasitism results in poor host plant growth and presents a serious threat to the production of many important horticultural and field crops [107–109]. As countermeasures, several means with nematode-suppressive properties have been reported, such as applications of compost with a low C/N ratio (< 20) [110, 111], volatile fatty acids [112], chitin [113], and plant-specific toxins [114]. A few studies also showed that application of biogas digestate to soil reduced the root gall formation of root-knot nematodes of tomato [115] and the damage to sugar beet by *Heterodera schachtii* [116].

A recent study showed that populations of *M. incognita* did not decrease in soil added with dry biogas digestate (C/N ratio of 20) treatment, compared with those in chemical fertilizer treatment [82]. Several studies have already reported that not all types of organic amendments are beneficial in the suppression of root-knot nematodes [117, 118]. For instance, Bulluck [117] also observed that

## *Dry Anaerobic Digestion for Agricultural Waste Recycling DOI: http://dx.doi.org/10.5772/intechopen.91229*

*M. incognita* populations were not affected by amendments of swine manure and composts. There are several factors which determine the effect of organic fertilizer on plant-parasitic nematodes, and the most commonly reported one is C/N ratio [119]. Organic amendment with a C/N ratio in the range of 15–20 was considered most effective [114]. In a study by Agu [120], plants of African yam bean treated with poultry and farmyard manures (C/N ratio of 4 to 12) showed a lower degree of disease caused by root-knot nematodes than those with other organic manures with C/N ratios higher than 30. In the study by Wang [82], the populations of *M. incognita* drastically decreased in Mix 2 treatment, in which biogas digestate was co-added with rice straw to increase its C/N ratio from 12 to 30.

Organic amendment may have different effects on different soil microbial groups, and nematodes could be reduced by such a modified microbial group [119, 121]. The prokaryotic community structure of the treatments reported by Wang [82] was evaluated, and the results showed that Mix2 treatment, in which low NO3 <sup>−</sup> risk and high nematode suppression were confirmed, was separated from the other treatments, indicating that a specific microbial community was developed in the treatment (**Figure 2**). Several papers have already reported that the application of biogas digestate affected the community structure of bacteria and fungi [122–124]. In general, organic amendment stimulates a broad range of (micro) organisms involved in the soil food web, many of which are potential predators, such as diplogasterid [125] and dorylaimid [126], or invertebrate antagonists, such as enchytraeids and earthworms [127]. Moreover, nematode suppression might result from increased incidences and levels of nematode-antagonistic fungi following amendment application. According to Wang [128, 129], the application of sunn hemp crop residues to soil decreased the population levels of the plant-parasitic nematode *Rotylenchulus reniformis* and increased levels of nematode-trapping fungi, such as *Arthrobotrys oligospora* [130] and *Ematoctonus leiosporus* [131]. The mode of action in biogas digestate leading to nematode suppression and stimulation of microorganisms is complex and dependent on the nature of the original wastes. Therefore, long-term use of biogas digestate to build suppressive elements of the soil food web remains an elusive goal.


### **Figure 2.**

*A Uni-Frac weighted PCA analysis of prokaryotic communities of soils with different amendments and incubated for 90 days. NF: no fertilizer, CF: chemical fertilizers, DryBD20: dry biogas digestate with an C/N ratio of 12, DryBD30: dry biogas digestate with an C/N ratio of 16, Mix1: DryBD20 mixed with a low amount of rice straw to adjust its C/N ratio to 16, Mix2: DryBD20 with a high amount of rice straw to adjust its C/N ratio to 30.*
