The Biosphere: Biogeochemical cycling of C,N,P in terrestrial systems

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1 The Biosphere: Biogeochemical cycling of C,N,P in terrestrial systems Outline: Introduction Biogeochemical cycling in land plant o Nutrient uptake o Nutrient balance o Nitrogen assimilation o Nitrogen fixation o Mycorrhizal fungi Nutrient allocation and cycling in land vegetation o Annual intrasystem cycle o Litterfall Biogeochemical cycling in soils Please note that the majority of the text below is taken from Chapter 6 in: Schlesinger, W.H Biogeochemistry: an analysis of global change. 2 nd edition. Academic Press, California. Introduction In any terrestrial ecosystem, receipt of elements from the atmosphere and lithosphere represents an input of new quantities of nutrients for plant growth However, because of internal cycling and retention of past inputs, plant growth not solely dependant on new inputs to system Annual circulation of important elements such as N within ecosystem is often times greater than amount received from outside system (Table 6.1) o This large internal, or intrasystem cycle, is achieved by long-term retention of elements received from atmosphere and lithosphere Important biochemical elements are accumulated in terrestrial ecosystems by biotic uptake, whereas nonessential elements pass through systems under geochemical control Biogeochemical cycling in land plants Nutrient uptake Soil chemical reactions, such as ion exchange and mineral solubility, set initial constraints on availability of essential elements for plant uptake When plant uptake of an element such as P is rapid, additional P may enter the soil solution from dissolution of minerals o Plants can release organic compounds that enhance solubility of various nutrients from soil minerals.

2 Delivery of ions to plant roots can occur by several pathways: 1. Passive uptake with water 2. If delivery is excessive, ions may be actively excluded at the root surface 3. For N, P and K, concentrations in soil solution often much to low for adequate delivery in transpiration stream, so plant uptake enhanced by enzymes that carry ions across root membrane using active transport Uptake of N and P so rapid, and concentrations in soil solution typically so low, that these elements are effectively absent in soil solution surrounding roots Rate of uptake determined by diffusion to the root from other areas Phosphate is particularly immobile in most soils, so rate of diffusion strongly limits its supply to plant roots Most apparent response of plants to low nutrient concentrations is an increase in root:shoot ratio This increases volume of soil exploited and decreases diffusion distance In many species, relative growth rate of roots determines uptake of N and P (e.g., Figure 6.2) Roots show rapid proliferation in nutrient-rich soils Higher plants and soil microbes exude enzymes into soils that can release inorganic P from organic matter These enzymes (phosphatases) have different forms in acid and alkaline soils o Root phosphatase activity usually inversely proportional to available soil P Phosphatase activity associated with root surfaces is particularly significant to plants in P- poor habitats May supply up to 69% of annual P demand of some tundra plants Nutrient Balance Plant growth is affected by balance of nutrients in the soil However, unless supply of a nutrient reaches very low levels, plants usually do not show deficiency symptoms o They simply grow slower Inherent slow growth is a characteristic of plants adapted to infertile habitats o Often persists even when nutrients are added experimentally Because more nutrients occur as positively charged ion than as negatively charged ones in soil solution, one might expect that plant roots would develop a charge imbalance as a result of nutrient uptake When ions such as K + are removed from soil solution in excess of the uptake of negatively charged ions, the plant releases H + to maintain an internal charge balance

3 o This H + may replace K + on a cation exchange site, driving K + into soil solution The high concentration of N in plant tissues causes the form in which N is taken up to dominate this process (e.g., Table 6.2) Plants that use NH 4 + as a N source tend to acidify the immediate zone around their roots Uptake of NO 3- has opposite effect as a result of plant releases of HCO 3- and organic acids to balance negative charge. Nitrogen Assimilation Among various habitats, availability of soil nitrogen as NH 4+ or NO 3- differs largely depending on environmental conditions that affect conversion of NH 4+ to NO 3- via microbial nitrification E.g., in waterlogged tundra soils, almost all N is found as NH 4+, whereas in some deserts and forests, only NO - 3 is important o Many plant species show a preference for NO 3-, although species occurring in sites where nitrification is slow or inhibited often show superior growth with ammonium Inside the plant, both NH 4+ and NO 3- are converted to amino groups (-NH 2 ) that are attached to soluble organic compounds In many woody species, conversions occur in roots, and N is transported to the shoot as amides, amino acids and ureide compounds in the xylem o In some species, N in xylem found as NO 3- and reduction of NO 3- to NH 2 (which requires metabolic energy and the enzyme nitrate reductase) occurs in leaf tissues Eventually, most plant N incorporated into protein Nitrogen Fixation Several types of bacteria and blue-green algae possess the enzyme nitrogenase, which converts atmospheric N 2 to NH 3 Some exist as free-living (asymbiotic) forms in soils Others, such as Rhizobium and Frankia, form symbiotic associations with roots of higher plants (usually reside in root nodules) In both symbiotic and asymbiotic forms, N-fixation is generally inhibited at high levels of available nitrogen In many cases, rate of fixation appears to be controlled by N:P ratio in soil o e.g., added P stimulates asymbiotic N-fixation in prairie soils (Figure 6.3) In bacteria, P appears to activate gene for synthesis of nitrogenase

4 o Requirements for Mo and Fe as structural components of nitrogenase also link N-fixation to availability of other elements in system! Some plants with symbiotic N-fixing bacteria appear to acidify their rooting zone to make Fe and P more available Asymbiotic bacteria and blue-green algae are widespread, and their N-fixation can be important source of N for some terrestrial ecosystems However, in most regions, total N input from asymbiotic fixation is about equivalent to annual deposition of N in wet and dryfall from atmosphere. Mycorrhizal Fungi Symbiotic associations between fungi and higher plants found in most ecosystems Because of their large surface area and efficient absorption capacity, Mycorrhizal fungi are able to obtain soil nutrients and transfer these to the higher plant root via hyphae that actually penetrate cells of the root Mycorrhizal fungi also directly involved in the decomposition of soil organic materials through release of extracellular enzymes like cellulases and phosphatases and in weathering of soil minerals through release of organic compounds Most of these reactions are associated with plant roots; mycorrhizae simply enhance their occurrence in the rhizosphere, increasing overall rate of nutrient uptake In return, fungi depend on host plant for supplies of carbohydrates o Mycorrhizal fungi are especially important in transfer of those soil nutrients with low diffusion rates in soil.! Large numbers of studies document importance of mycorrhizae in P nutrition, but absorption of N and other nutrients is also known Nutrient Allocations and Cycling in Land Vegetation Annual Intrasystem Cycle Uptake of nutrients from soil is allocated to growth of new plant tissues Although short-lived tissues (leaves and fine roots) compose only small fraction of total plant biomass, they receive largest proportion of annual nutrient uptake o e.g., Growth of leaves and roots received 87% of N and 79% of the P allocated to new tissues in a deciduous forest in England When leaf buds break and new foliage begins to grow, leaf tissues often have high concentrations of N, P, and K As foliage matures, initial concentrations of N and P are diluted as leaf tissues accumulate carbohydrates and cellulose o In contrast, concentration of some nutrients (Ca, Mg, and Fe) often increases with leaf age because of deposition and storage in plant tissues

5 Nutrient concentrations in mature foliage are related to rate of photosynthesis and plant growth Leaf concentrations of trace metals often reflect content of underlying soil Leaf content of N, P and K remains relatively constant at high levels throughout the growing season, but was strongly removed from leaves in autumn This is active withdrawal of nutrients from foliage for reuse during next growing season o Usually readsorption of Ca and Mg is limited Rainfall may also leach nutrients from leaf surface (e.g., K which is highly soluble and especially concentrated near the leaf surface Losses of nutrients in leaching often follow the order: K>>P>N>Ca Leaching rates generally increase as foliage senesces before abscission Rainwater that passes through tree canopy called throughfall, and contains nutrients leached from leaf surfaces Litterfall In the intrasystem cycle, plant litterfall is dominant pathway for nutrient return to soil, especially for N and P (Figure 6.6) Below ground, root death also makes major contribution of nutrients to soil each year Nutrient concentrations in litterfall differ from nutrient concentrations in mature foliage by reabsorption of constituents during leaf senescence Nutrient reabsorption potentially confers a second type of nutrient-use efficiency on vegetation o Nutrients that are reabsorbed can be used in NPP in future years, increasing carbon fixed per unit of nutrient uptake. A mean fractional reabsorption of 50% N and 52% P during leaf senescence was found among a wide range of species Plants grown with low nutrient availability or occurring on infertile sites tend to have low nutrient concentrations in mature leaves and litter They generally reabsorb a smaller amount, but a larger proportion, of the nutrient pool in senescent leaves compared with individuals of the same species under conditions of greater nutrient availability

6 Differences in nutrient-use efficiency in reabsorption between nutrient-rich and nutrientpoor sites are not likely due to a direct response of plants, but to tendency for species with higher inherent capabilities for nutrient reabsorption to dominate nutrient-poor sites. Biogeochemical Cycling in the Soil Most of annual nutrient requirement of land plants is supplied from decomposition of dead materials in soil Decomposition: term referring to breakdown of organic matter Mineralization: more specific term referring to processes that release carbon as CO 2 and nutrients in inorganic form (e.g., P as PO 4 3- ) A variety of soil animals (e.g., earthworms) fragment and mix fresh litterfall However, main biogeochemical transformations performed by fungi and bacteria in soil Total microbial biomass (bacteria and fungi) typically composes <3% of organic carbon found in soils Soil microbes have high nutrient concentrations relative to organic matter they decompose o e.g., Contained % of organic C, but up to 19.2% of organic P in tropical soils of India Respiration of soil microbes converts organic C to CO 2, while the N and P are retained in microbial biomass The accumulation of N, P and other nutrients in soil microbes is known as immobilization. o Immobilization most significant for N and P, which are limiting to microbial growth, and usually less obvious for Mg and K, which are available in greater quantities Microbes also accumulate nutrients from soil solution (Figure 6.9) o Microbial uptake of NH 4+ is rapid, sequestering available NH 4+ that might otherwise be available for plant uptake of nitrifying bacteria During decomposition, a fraction of substrate is converted to fulvic and humic compounds that have high N content and long-term stability in soil Decaying plant litter appears to adsorb Al and Fe, perhaps in compounds that are the precursors to the fulvic acids that carry Al and Fe to lower soil profile via podzolization When microbial activity slows, little nutrient immobilization When substrate exhausted and microbes die, N is released as NH 4+ from dead microbial tissue

7 Plant litter with high concentrations of nutrients decomposes more rapidly, and net mineralization is likely to begin earlier Fallen logs, though, have low N contents, and long-term immobilization of N is especially evident during log decay Ecologists have long used the C/N ratio of litterfall as index of potential rate of decomposition Lignin/N ratio in litterfall also predictor of rate of decomposition in various ecosystems (Figure 6.10) Immobilization of nutrients predominates in layer of fresh litter, while mineralization of N, P and S is usually greatest in lower forest floor Release of N, P and S from soil organic matter likely to occur as different rates Table 6.8 shows mean residence time for organic matter and its nutrient content in the surface litter of various ecosystems Pool of soil organic matter greatly exceeds mass of live biomass in most ecosystems o Because of high nutrient content, humus also dominates the storage of biogeochemical elements in most ecosystems.