Biosolids, Yield, Organic amendments in SOil. research to mitigate LeachIng and Denitrification: BYOSOLID

Work at Rothamsted strongly suggests that by adding organic matter to soil, crop yields can be improved over and above any response to the nutrients that the amendments contain. The mechanism appears to be that soil organisms feeding off the amendments improve soil structure and thus allow plants to establish and find water and nutrients more easily. Crops grown in amended soils do especially well in years when crop yields are impacted by dry springs or wet winters. Work is underway at Rothamsted to better understand the mechanisms behind these observations. This project seeks to extend this knowledge to the duration of the benefit: how many years after an amendment has been added can the benefits to yield still be seen and how great might these be?
There are many materials that are or could be added to soil and which ought to increase yields in this way: manure, compost, anaerobic digestate (AD), biosolids. Recycling these materials on land makes sense since they are all derived from agriculture in one way or another. However, there are regulations associated with their use. EU regulations restrict applications of materials containing N and P in Nitrate Vulnerable Zones and if a material is defined as a 'waste' (and some of these are) then the licensing issue becomes one of disposal rather than recycling. Current projects at Rothamsted have as their aims to produce guidelines to farmers on the use of organic amendments to improve crop yields. Because of the risk of environmental pollution from these materials there is a risk that regulatory agencies might refuse to permit the use of organic amendments or restrict their use still more than at present. This project aims to quantify not only the risk of N emissions from the direct application of amendments but also the residual release in the years subsequent to application.
The project will examine emissions of N in a number of experiments currently receiving amendments, experiments that will cease to receive amendments during the course of the project and experiments which received amendments historically but which ceased during the last 10 to 15 years. In all cases, we know we can find the plots and have records of land management since amendment. Thus we expect to be able to build up knowledge of the loss of nutrients from land that last received amendment between 0 and 15 years ago on a range of soil types for several different kinds of amendment. Gaps will be filled by interpolation using computer simulation models. Such a comprehensive study should help mitigate the risks of restrictions being placed on the recycling of organic materials to land or provide objective evidence in support of their beneficial use or proscribe the extent of any restriction.
Fertiliser N is most economically applied at the Break-Even Ratio (BER) - where application of one more increment of fertiliser only just pays for one more increment in yield. Organic amendments at Rothamsted appear to change the response of crops to added N and hence the N at BER and the amount of yield expected. Benefits from amendments will be expressed in terms of the change in position of the BER relative to yield and fertiliser N applied and in terms of the cost of fertiliser and the value of product.
APW has been in touch with Anglian Water about this project and with HGCA, both of whom could be major beneficiaries. PW has also talked to the composting industry about the potential results.

The average UK citizen produces sewage containing between 6-8 kg N yr-1 or nationwide about 400 kt N (Addiscott et al., 1991). Domestic food waste in the UK is about 7.2M t yr-1, other sources double this. Taking a broad average water content as 50% and N content as about 2%, such waste which could end up in compost or AD contains about 145 kt N. Taken together the biosolids and food waste contain as much N as is found in half of the UK's annual consumption of N fertiliser of 1Mt. It is true that not all food wasted nor all UK sewage is produced from UK fertiliser, but neither is all the food produced in the UK consumed in the UK. It is also true that the N contained in an organic material is no guide to availability nor is it a guide to amount that it would be economic or feasible to apply. A report to the Scottish Government (2009) estimates that feasible use of biosolids was worth 35 kg N ha-1 to agriculture Another key statistic is that the soil nutrient balance (surplus) for the UK is 92 kg ha-1 (Defra, 2013) which is a surplus of 1.1 Mt on a managed area of about 12 Mha or 17 kg N per UK citizen. The UK is thus gaining, not exporting N, suggesting a great need to recycle.
In terms of carbon, 2.52 GJ energy is needed to produce 1 tonne of bread-making wheat in the UK; almost half of this is related to N fertiliser production (Woods et al 2010). Clearly re-use of fixed N through recycling could also save substantial amounts of fossil fuels and sensible re-use of fixed N could be of considerable environmental benefit provided that doing so reduces overall emissions of N.
There could also be general public good from better recycling since it would reduce the expense and pollution that results from the disposal of materials such as biosolids by burning or landfill.
Those bodies responsible for managing materials, such as water and sewerage companies or large produce companies who have crop residues to deal with, should benefit, via a reduction in the risk of failure to obtain licences or permissions to apply, from better information about the emissions and risk of emission that recycling of their materials brings.

References
Addiscott et al. 1991 Farming Fertilizers and the Nitrate problem CABI pp170

Defra 2013 Soil Nutrient Balances UK Provisional Estimates for 2012

Scottish Government 2009 http://www.scotland.gov.uk/Publications/2009/01/08100107/15

Woods, J et al. PToRS 2010 365, 2991