Strategies to reduce losses and improve utilisation of nitrogen from solid cattle manure

Background and objectives

The number of domesticated cattle in the world has steadily increased during the last decades, and thereby also the amount of manure produced annually. The excrements of grazing cattle are dropped in pastures and left unmanaged, but that of confined and housed cattle are collected and managed. The collected manure is often a variable mixture of urine, faeces, bedding material and spoiled feed and (drinking) water. On most modern farms, excrements are usually collected in leak-tight storages and handled as slurry: a mixture of urine, faeces and spoiled water. However, on a significant fraction of farms, cattle excrements are ‘source-separated’ in a liquid fraction and a solid fraction. The solid cattle manure (SCM) is usually a mixture of faeces and bedding material with some absorbed urine. The production of SCM is increasing due to the renewed interest in straw-based housing systems for better animal health and welfare. It has been observed that a significant loss of N can occur, especially from the storage and application phases of the SCM management chain. This N loss pollutes the air, groundwater and surface waters, and also reduces its N fertiliser value. Thus the challenge is to develop an effective SCM management system that retains as much of the excreted N in the system as possible, and thereby improving on-farm N cycling through the cattle-manure-soil-crop continuum (Chapter 1). Themain objective of this PhD thesis research was to increase the understanding of the factors controlling N losses during storage and after field application, and to develop and test strategies to decrease N losses and improve crop utilisation of N from SCM. The specific objectives were:

To study the interactions between a number of animal manures and soil types on N mineralisation and plant N recovery (Chapter 2) To investigate the effects of storage conditions on (i) magnitude and pathways of C and N losses during storage of SCM, and (ii) crop apparent N recovery (ANR) and DM yield (Chapter 3) To examine manure disappearance rates, N release pattern and herbage ANR during the year of application and the year thereafter from surface applied SCM subjected to different storage conditions (Chapter 4), and To analyse the effect of various application strategies on NH₃ emission and/or crop ANR from applied SCM to grassland and arable (maize) land (Chapters 3 and 5)

To pursue these objectives a pot experiment in a glasshouse (Chapter 2) and a number of field experiments (Chapters 3 to 5) were conducted on experimental facilities of Wageningen University, the Netherlands. The pot experiment dealt with net N mineralisation and herbage ANR from SCM, cattle slurry and poultry manure, all applied to peat, sandy and clay soils. The field experiments examined (i) total C and N losses from stockpiled, composted, covered and roofed SCM heaps, (ii) manure decomposition, N release and herbage ANR after surface application of fresh and stored SCM on grassland, and (iii) the effects of irrigation and soil incorporation after SCM application, and lava meal as an additive on NH₃ emission and/or crop ANR by grassland herbage or arable maize.

 

Major findings of the thesis

Results of the pot experiment showed that net N mineralisation and herbage ANR varied as function of manure storage method and soil type. Irrespective of the manure types, net N mineralisation and herbage ANR were highest in peat soil, which was characterised by the greatest N delivering capacity. Between the clay and sandy soils, both having similar N delivering capacity, net N mineralisation and herbage ANR were lower in the clay soil than in the sandy soil, likely because of immobilisation and fixation of ammonium-N by its inherited higher clay content. On each soil type,ANR was lower from SCM than cattle slurry and poultry manure(Chapter 2). The N recovery fraction was low when SCM was stored traditionally (i.e. stockpiling or composting) due to (i) loss of the initial mineral N content and readily degradable organic N compounds, and (ii) conversion of part of the remaining N into more stable forms as compared to that originally present before storage. Up to 31% of the initial total N from the stockpiled and 46% from the composted SCM heaps were lost during a period of about four months. Covering and roofing of SCM heaps reduced the losses down to 6 and 12%, respectively. Of the total N losses from each storage method, only about one fourth could be traced back as NH₃-N and N₂O-N emissions, and/or N leaching. The remainder could not be accounted for and constituted, in all probability, of harmless N₂ gas. Of the total measured gaseous and liquid N losses together, N leaching contributed the most. The leaching N losses were reduced by almost three times through protection of SCM heap against precipitation either by its covering or roofing when compared to its stockpiling or composting in the open air. Although stockpiling of SCM under a roof significantly reduced overall total N losses, NH₃ and N₂O emissions were much higher as compared to stockpiling of SCM in the open air. Composting of SCM resulted in higher gaseous N emissions as well as N leaching with respect to the other storage methods. In view of these finding I conclude that covering of SCM heaps with an impermeable sheet is the best option to reduce storage N losses (Chapters 3 and 4).        

            In addition, because of N conservation and slow mineralisation of the organically bound N during the covered storage, mineral N content of SCM increased at the end of the storage phase. This, together with high mineralisation activities after field application of covered SCM, led to greater crop ANR and DM yield especially when compared to composted SCM, both in the year of application and in the subsequent year. When N losses during storage was taken into account to arrive at the crop ANR of the collected manure from the barn, it turned out that the ANR value was about three times larger in case of covered storage compared to composting of SCM, both for grassland (21 vs. 7%; Chapter 4) and arable land (37 vs. 13%, Chapter 3). Interestingly, despite of some N losses during covered storage (~10% of the initial N), crop ANR and DM yield were significantly larger from covered than fresh SCM taken directly from the barn, again in both situations.

            Irrigation immediately after SCM spreading and use of lava meal as an additive significantly (i) reduced NH₃ emission and (ii) improved crop ANR as well as DM yield (Chapters 3 and 5).Irrigation at a level of 5 mm immediately after surface application of fresh and covered SCM to grassland reduced NH₃ emission by 30 and 65%, respectively, whereas it was not effective in case of composted SCM, likely because of its greater DM content. Addition of lava meal before application at a rate of 80 g per kg of covered SCM resulted in an emission reduction of 46%. By combining it with 10 mm irrigation, an almost 100% reduction in NH₃ emissions from covered SCM was realised, whereas herbage ANR increased from 18 to 26% of the applied N over a growing period of five months (Chapter 5). Incorporation of SCM just before sowing of maize resulted in an ANR value of 39% from covered SCM, whereas this fraction was 20, 29 and 31% in case of composted, stockpiled and roofed manure, respectively (Chapter 3).

                                                                                                          

Overall conclusions

The ANR from applied manure in harvested herbage depends on manure type and soil type, and varies widely. It is lower from SCM than from cattle slurry Total N losses during storage of SCM can be reduced remarkably by covering the heap with an impermeable sheet. Covering reduced two N loss pathways: (i) gaseous N emissions to air, and (ii) N leaching to surface waters and groundwater. Field application of SCM that was covered by a sheet during storage, decomposed faster and more N was available for plant uptake, both in the year of application and the subsequent year, when compared to SCM that was stored in traditional ways Emission of NH₃ following land application of SCM can be reduced greatly by irrigation or incorporation immediately after SCM spreading, and using lava meal as an additive. Irrigation appeared to be more effective in reducing NH₃ emission than the addition of lava meal. All these NH₃ emission abatement measures substantially increased crop ANR and DM yield Overall, combining covered storage with either direct irrigation following application of SCM to vegetated soil or direct incorporation in the soil following application of SCM to arable land is the best practical option to reduce losses and improve utilisation of N from SCM management systems. Depending on the farm infrastructure, losses may be further reduced by the use of lava meal, preferably as a bedding additive in the barn

 

Implication for efficient manure management

In many industrialised countries, animal manure is a major source of environmental pollution. In contrast, in most of the developing countries animal manure is considered as a key nutrient source to maintain or improve crop productivity and therefore N losses from manure management are more seen as ‘loss of plant nutrient’ rather than ‘pollution problems’. In either case development of efficient SCM management systems is highly important. Based on the results of this thesis, I propose some key management actions to improve the agro-environmental value of SCM.

If economically attractive, apply lava meal to straw bedding in the barn (Chapter 5) Store the barn-produced SCM under impermeable sheet (Chapters 3 and 4) Crop and soil-specific SCM application rates must take into account the potential available N (Chapter 2) and degradability of organic N compounds (Chapter 4) Incorporate the SCM from covered storages directly into the soil when applied to arable land (Chapter 3) In situations where incorporation is not feasible, like on grassland, spread SCM just before a predicted rainfall event or apply irrigation otherwise (Chapter 5) Take into account the expected residual N contribution from earlier manure input when determining the manure application rate(Chapter 4)