Relevance of changes in soil quality for ecosystem services

Soil biota are key for a range of ecosystem services, including soil structure mainte- nance, water retention, supply of nutrients and grass production (Swift et al., 2004; Mulder, 2006; Brussaard et al., 2007a; Kibblewhite et al., 2008). Soil structure maintenance is an ecosystem service provided by SOM, roots and soil biota such as earthworms, bacteria and fungi. Under pure grass, with a higher N application rate, Van Eekeren et al. (2009a) found some evidence that solid organic manure contributed more to soil structure maintenance than inorganic fertilizer. Under clover-only, Van Eekeren et al. (2009b) found a less developed soil structure than in pure grass or the mixture of grass-clover. In our experiment, with a tendency of reduced SOM-levels over the years but no significant treatment effects on root, earthworm and microbial parameters, only little effect was measured in 2007 on the soil physical param- eters underlying the service of soil structure maintenance. Compared to no manure, the bulk density was lower in FYM and NSM, and was positively correlated with clover content. The latter is in line with Van Eekeren (2009b). Our results suggest that after seven years, with a tendency of reduced SOM levels, there was only little effect of N application rate and manure type on the ecosystem service of soil structure maintenance in grass-clover.

Water regulation is an ecosystem service largely provided by SOM, directly through water retention (Gupta and Larson, 1979; Ohu et al., 1987), and by earthworms through their burrowing activities, which stimulate water infiltration and deeper root growth (Hoogerkamp et al.,1983; Clements et al., 1991; Logsdon and Linden, 1992; Edwards and Shipitalo, 1998). Therefore, the higher SOM content in FYM in our experiment is expected to lead to a higher water retention. The number of earthworm burrows was not significantly different between the treatments. Van Eekeren et al. (2009a) measured more earthworm burrows under grass

in plots with organic fertilizers compared to the unfertilized control, and Van Eekeren et al. (2009b) found more earthworm burrows under clover. This suggests that clover in this experi- ment compensated for the effects of a reduced manure application on earthworm burrows. Moreover, the soil penetration resistance did not differ in the treatments, indicating no change in earthworm activity. All together the results suggest that the higher SOM in FYM treatments can increase the water retention but that no differences in water infiltration are expected.

Concerning the ecosystem service of nutrient supply, the highest N application led to the highest SOM, the highest pH, the highest bacterial activity and the highest potentially min- eralizable C. However, these treatment effects did not result in significant differences in the grass-clover yield over the years 2003 and 2007. Grass-clover yield was not only positively correlated with bacterial activity, but also with the clover content in the sward, which is an important determinant for N fixation. Baars (2002) found higher grass-clover yields in treat- ments that received slurry and farm yard manure than in a no-manure treatment. He ascribed this partly to a shortage in potassium in the non-manure treatment. Since in our experiment the potassium and phosphate fertilization for all treatments were the same, only the N application could have made a difference in grass-clover yield. Considering the effect of N application on grass-clover, Schils and Snijders (2004) found that the increased N yield due to N application (up to 190 kg N ha-1_{) was completely offset by the reduced N fixation. Both Schils (1997)}

and Baars (2002) argue that N application before the first harvest results in higher DM yield. In our experiment such differences could not be measured. However, we found a decrease in DM yield from 2003 to 2007, which was probably related to the lower clover content in 2007 compared to 2003. The very dry summer of 2003 reduced the clover content at the end of the summer from which it did not recover to the dominance in 2003. It is known that it is difficult to maintain an optimal clover content in accordance to the nitrogen supply capacity of the soil. Management measures to maintain an optimal clover content like choice of a persistent clover cultivar, and an optimal potassium and phosphate supply (Baars, 2002; Van Eekeren et al., 2005b), were taken in this experiment. However, the pH at the start of the experiment was too low for clover. To optimize the pH to the official Dutch fertilization recommendations of grass-clover (pH-KCl 5.2-5.5; http.//www.bemestingsadvies.nl), lime was applied. Pos- sibly, the increase in pH with a subsequently increased bacterial activity, led to an increased N mineralization and N availability, which had a negative effect on the clover content. In experiments of Scheu (2003) and Kreutzer et al. (2004) an increased availability of N by min- eralization, through the presence of earthworms, counteracted the dominance of T. repens by increasing the biomass of L. perenne. The higher N mineralization must have increased the grass production but did not completely compensate for the loss of N availability by fixation of the clover. Based on a theoretical N fixation of 39 g N 1000 g -1 _{white clover according to}

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Chapter 5 - Manure types and N application rates on grass-clover 123

Therefore, we conclude that under grass-clover the ecosystem service of nutrient supply of N is dominated by the N-fixing Rhizobium bacteria, which depends on the clover content in the sward. Besides other management options to maintain the clover cover (e.g. per- sistent clover cultivar, and an optimal potassium and phosphate supply), the initial pH when seeding grass-clover should be within the limits of the recommendations. In the following years the pH should be maintained on a yearly basis to compensate for acidifying effects of clover accounting for the grazing and cutting management, and organic manure application.

5.5

Conclusions

We conclude that with the export of animal manure from organic dairy farms to or- ganic arable farms the soil biological quality under grass-clover pastures will be sustained by the clover if it persists in the grass-clover sward. Most effects of manure type and N applica- tion rate on soil biological quality were compensated for by the N fixation of the clover. How- ever, even with the application of organic manure, there was a tendency towards a decrease in SOM, whereas normally there is a net accumulation. One of the causes was probably liming, which increased pH, bacterial activity and mineralization. It is clear that the whole system depends on the clover persistence. Over the years a reduction in clover content resulted in a re- duced DM yield, increased abundance and percentage of herbivorous nematodes, decrease of bacterivorous nematodes and increase in fungal biomass. It is likely that, amongst others, an increased N mineralization due to liming had an overall negative effect on the clover content. The higher mineralization must have increased the production but not completely compensat- ed for the loss of the N fixation of the clover. A persistent white clover cultivar in combination with a soil pH within the limits of the recommendations before sowing, should guarantee the persistence of the clover, next to an optimal fertilization of potassium and phosphate.

Acknowledgements

We thank the staff of the ”Aver Heino” experimental research farm for their contribu- tions to the field work. We thank Riekje Bruinenberg, Jan Bokhorst, Popko Bolhuis, Bert van Dijk, Erik Steenbergen, Meint Veninga and An Vos for their assistance with soil sampling and the analyses of the different parameters. Jan-Paul Wagenaar and Dré Nierop are acknowledged for their assistance with data analysis. The experiment was conducted under the Bioconnect

research programme financed by the Ministry of Agriculture, Nature and Food Quality. The Dutch Soil Quality Monitoring Network made it possible to carry out additional soil biotic measurements. The microbiological work was supported by the research programs BO-07- 010 “Agrobiodiversity”, BO-001-002 “Soil” and KB-01 “Sustainable spatial development of ecosystems, landscapes, seas and regions”.

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Chapter 6 - Grass species and grass mixtures 127

Chapter 6

Effects of individual grass species and