6.2: Nutrient allocations and cycling in land vegetation

Table \(\PageIndex{1}\): Management actions affecting soil nutrient cycles and their impact on ecosystem services.

Management action or other driver of change	Provisioning service impact	Regulating service impact	Supporting service impact	Cultural service impact
Intensive addition of mineral fertilizers	Increased food, fibre, and feedstock production	Reduced water quality through eutrophication, reduced air quality through emission and volatilization of reactive N gases	Increased primary production; alteration of the nutrient and C cycling; possible reduction of biodiversity
Use of organic soil amendments (e.g. manure, composts and biochar)	Increased food, fibre, and feedstock production; may increase water retention	Increase C sequestration	Increase nutrient retention
Implementation of no-till			Increase nutrient retention
Precision agriculture	Increase efficient production of food	Reduced GHG emissions per unit production	Reduce consumption of water and nutrient by improving use efficiency
Prescribed use of fire for pasture management	Increase feedstock production	Increase C sequestration by conversion to BC	Reduce N recycling by storing black nitrogen
Use of biological soil supplements	Stimulate productivity; act as fertilizers	May improve pest and disease control	Improved nutrient cycling

Soils support primary production among other services, which in turn delivers the provisioning services of food and fibre production (Table \(\PageIndex{1}\)). As such, soils are vital to humanity since they provide essential nutrients, such as nitrogen \((\mathrm{N})\), phosphorus \((\mathrm{P})\), and potassium \((\mathrm{K})\) and many trace elements that support biomass production, which is essential for the supply of human and animal food, for energy and fibre production and (future) feedstock for the chemical industry (Table \(\PageIndex{1}\)). Since the 1950s, higher biomass production and yield increases have been supported by fertilizers derived from mined minerals or industrial synthesis (Fig. \(\PageIndex{1}\)). Intensification of agricultural practices and land use has in many regions resulted in a decline in the content of organic matter in agricultural, arable soils (Table \(\PageIndex{1}\); Matson et al., 1997; Smith et al., 2015). In some areas, extensive use of mineral fertilizers has led to atmospheric pollution, greenhouse gas emissions (e.g. \(\mathrm{N}_2 \mathrm{O}\), very important for climate regulation), water eutrophication, and human health risks (Galloway et al., 2008), thereby negatively affecting the regulating services of soil, air, and water quality (Table \(\PageIndex{1}\); Smith et al., 2013). During the 21st century, it is likely that the human population and demand for food, feed, and energy will rise. In order to sustain biomass production in the future, and to avoid negative environmental impacts, fertile soils need to be preserved and soil fertility needs to be restored where lost. This can be done through both the recycling and accumulation of sufficient amounts of organic matter in soils (Janzen, 2006), through a combination of plant production and targeted additions of organic and mineral amendments to soils.

Figure \(\PageIndex{1}\):Global (a) nitrogen (N) and (b) phosphorus (P) fertilizer use between 1961 and 2012 split for the different continents in Mt P per year; plotted from FAOSTAT data (FAOSTAT, 2015). (Copyright: CC-BY, P. Smith, et al., SOIL https://soil.copernicus.org/articles/1/665/2015/)

The soil function "fertility" refers to the ability of soil to support and sustain plant growth, which relates to making available N, P, other nutrients, water, and oxygen for root uptake. This is facilitated by (i) their storage in soil organic matter, (ii) nutrient recycling from organic to plant available mineral forms, and (iii) physical-chemical processes that control their sorption, availability, displacement, and eventual losses to the atmosphere and water (Table \(\PageIndex{1}\)). Managed soils are a highly dynamic system and it is this very dynamism that makes the soil work and supply ecosystem services to humans. Overall, the fertility and functioning of soils strongly depend on interactions between the soil mineral matrix, plants, and microbes; these are responsible for both building and decomposing SOM, and therefore for the preservation and availability of nutrients in soils (Cotrufo et al., 2013). To sustain this service, the cycling of nutrients in soils must be preserved (Table \(\PageIndex{1}\)).

After carbon, \(\mathrm{N}\) is the most abundant nutrient in all forms of life, since it is contained in proteins, nucleic acids, and other compounds (Galloway et al., 2008). Humans and animals ultimately acquire their \(\mathrm{N}\) from plants, which on land is mostly taken up in mineral form (i.e. \(\mathrm{NH}_4^{+}\)and \(\mathrm{NO}_3^{-}\)) from the soil. The parent material of soils does not contain significant amounts of \(\mathrm{N}\) (most other nutrients such as \(\mathrm{P}\) largely originate from the parent material). New \(\mathrm{N}\) mostly enters the soil through the fixation of atmospheric \(\mathrm{N}_2\) by a specialized group of microorganisms. However, the largest flux of \(\mathrm{N}\) within the soils is generated through the continuous recycling of \(\mathrm{N}\) internal to the plant-soil system: soil mineral \(\mathrm{N}\) is taken up by the plant, is fixed into biomass, and eventually \(\mathrm{N}\) returns in the form of plant debris to the soil. Here microorganisms decompose it, mineralizing part of the \(\mathrm{N}\) and making it newly available for plant growth, while transforming the other part into SOM, which ultimately is the largest stock of stable \(\mathrm{N}\) in soil. Generally, \(\mathrm{N}\) cycles tightly in the system with minimal losses. Nitrogen is lost from the soil to the water system by leaching and to the atmosphere by gas efflux \(\left(\mathrm{NH}_4, \mathrm{~N}_2 \mathrm{O}\right.\), and \(\left.\mathrm{N}_2\right)\). In most terrestrial natural ecosystems, \(\mathrm{N}\) availability limits productivity. Through the cultivation of \(\mathrm{N}_2\) fixing crops, the production and application of mineral \(\mathrm{N}\) fertilizer, the increasing application of animal manure from livestock and bio-wastes, and the unintentional deposition of atmospheric reactive \(\mathrm{N}\) (ultimately derived from industrialera human activities), humans have applied twice as much reactive \(\mathrm{N}\) to soils as the \(\mathrm{N}\) introduced by natural processes, significantly increasing biomass production on land (Vitousek and Matson, 1993; Erisman et al., 2008). In some regions of the world, mineral fertilizer is applied in excess of plant requirement, but in other regions, in particular in Sub-Saharan Africa, where economic constraints limit the use of fertilizers, productivity is strongly limited by soil available \(\mathrm{N}\) and other nutrients, notably \(\mathrm{P}\) and \(\mathrm{K}\) ( \(\mathrm{N}\) and \(\mathrm{P}\); Fig. \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Applied and excess nitrogen and phosphorus in croplands. Nitrogen and phosphorus inputs and excess were calculated using a simple mass balance model (West et al., 2014), extended to include 175 crops. To account for both the rate and spatial extent of croplands, the data are presented as kilogram per hectare of the landscape. (a) Applied nitrogen, including N deposition; (b) applied phosphorus; (c) excess nitrogen; and (d) excess phosphorus. (Copyright: CC-BY, P. Smith, et al., SOIL https://soil.copernicus.org/articles/1/665/2015/)

Phosphorus derived from parent material, through weathering, cycles internally in the plant-soil system between biochemical molecules (e.g. nucleic acid, phospholipids) and mineral forms after decomposition (e.g. \(\mathrm{H}_3 \mathrm{PO}_4\) ). In soils, \(\mathrm{P}\) is among the most limiting of nutrients, since it occurs in small amounts and is only available to plants in its dissolved ionic forms, which promptly react with calcium, iron, and aluminium cations to form highly insoluble compounds. Largely in these forms, \(\mathrm{P}\) is lost to the aquatic system through erosion and surface runoff. Losses may also occur in dissolved form, for instance via subsurface flow and groundwater (McDowell et al., 2015). An important form of loss is in the export of organic \(\mathrm{P}\) in agricultural products. Due to widespread agricultural \(\mathrm{P}\) deficiencies, humans started to mine "primary" \(\mathrm{P}\) from guano or rock phosphate deposits and added it to soils in the form of mineral fertilizer (Fig. 2). This external input has led to positive agronomic \(\mathrm{P}\) balances (MacDonald et al., 2011) and excesses of \(\mathrm{P}\) and \(\mathrm{N}\) in many regions (West et al., 2014; Fig. \(\PageIndex{2}\)). There are large variations across the world, with high surpluses in the USA, Europe, and Asia and deficits in Russia, Africa, and South America (Fig. 3). Since plant \(P\) uptake is a relatively inefficient process with roughly \(60 \%\) of the total \(\mathrm{P}\) input to soils not taken up in the short term, a 3 -fold increase in the export of \(\mathrm{P}\) to water bodies has been estimated, with significant impacts on water quality (Bennett et al., 2001).

Clearly, management practices need to be implemented that sustain, restore, or increase soil fertility and biomass production by promoting the accrual of SOM and nutrient recycling, applying balanced \(\mathrm{C}\) amendments and fertilization of \(\mathrm{N}, \mathrm{P}\), and other nutrients to meet plant and soil requirements, while limiting the addition of excess fertilizer and retaining nutrients in the soil-plant system (Table \(\PageIndex{1}\)). C, N, and P cycling in soils is coupled by tight stoichiometric relationships (e.g. relatively fixed C : N : P in plants and microorganisms; Güsewell, 2004); thus their management needs to be studied in concert. Nutrient management has been extensively studied, with the aim of identifying and proposing management practices (e.g. precision agriculture) that improve nutrient use efficiency and productivity and reduce potentially harmful losses to the environment (Table \(\PageIndex{1}\); van Groenigen et al., 2010; Venterea et al., 2011). Yet, our ability to predict the ecosystem response to balanced fertilization is still limited, and effectiveness and reliability would benefit from continued monitoring of efforts. Further benefits are anticipated from improved plant varieties with root morphologies that have better capacity to extract \(\mathrm{P}\) from soils or use it more efficiently, perhaps in concert with mycorrhizal symbionts. Fertilization with nutrients other than \(\mathrm{N}\) and \(\mathrm{P}\) has been less well explored within the realm of understanding soil organic matter responses to agricultural \(\mathrm{C}\) inputs and the potential to restore and increase soil organic matter (e.g. Lugato et al., 2006). Hence, we stress the importance of an integrated approach to nutrient management, which supports plant productivity while preserving or enhancing SOM stocks, and reducing nutrient losses to the atmosphere or water resources. Several issues exist where prediction and optimization of performance would benefit from relevant and continued data acquisition for the range of climate and environmental and agro-ecological conditions. Table \(\PageIndex{1}\) summarizes some management actions affecting soil nutrient cycles and their impacts on ecosystem services.

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Excerpted from:

Smith, P., Cotrufo, M. F., Rumpel, C., Paustian, K., Kuikman, P. J., Elliott, J. A., McDowell, R., Griffiths, R. I., Asakawa, S., Bustamante, M., House, J. I., Sobocká, J., Harper, R., Pan, G., West, P. C., Gerber, J. S., Clark, J. M., Adhya, T., Scholes, R. J., and Scholes, M. C.: Biogeochemical cycles and biodiversity as key drivers of ecosystem services provided by soils, SOIL, 1, 665–685, https://doi.org/10.5194/soil-1-665-2015, 2015. https://soil.copernicus.org/articles/1/665/2015/