In terrestrial ecosystems, most nutrients regenerate in the soil

Now that we understand how the major nutrients cycle within and between ecosystems, we change our focus so we can examine how nutrients are regenerated in ecosystems. The regeneration of nutrients is a key factor in the productivity of ecosystems because the nutrients that are taken up by producers must be replaced. In this section, we will examine the regeneration of nutrients in terrestrial ecosystems, which experience nutrient regeneration by the weathering of bedrock or by the breakdown of organic matter.

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The Importance of Weathering

Terrestrial ecosystems experience a constant loss of nutrients because many are leached out of the soil and transported away in streams and rivers. To maintain a stable level of productivity, the loss of nutrients from an ecosystem must be balanced by an input of nutrients. For some nutrients, such as nitrogen, inputs come from the atmosphere. For most other nutrients, such as phosphorus, the inputs come from the weathering of bedrock beneath the soil. As we described in Chapter 5, weathering is the physical and chemical alteration of rock material near Earth’s surface. Substances such as carbonic acid in rainwater and organic acids produced by the decomposition of plant litter react with minerals in the bedrock and release various elements that are essential to plant growth.

Determining the rate of weathering can be difficult because bedrock often exists far below the surface of the soil. One solution has been to measure the nutrients that enter a terrestrial ecosystem from precipitation and the nutrients that leave an ecosystem by leaching out of the soil and into a stream. You can see a diagram of nutrient inputs and outputs in Figure 21.10. If the system is in equilibrium—if nutrient inputs equal nutrient outputs—the difference between the nutrients entering the system by precipitation and particulate matter and the amount of nutrients leaving the system by leaching and runoff should equal the amount of nutrients made available by weathering.

Figure 21.10 Quantifying nutrient weathering. Soil nutrients cycle through producers, consumers, and detritus. This cycle also has inputs from precipitation and weathering, and outputs of groundwater and surface water runoff.

Ecologists commonly determine the rate of nutrient regeneration through weathering by quantifying nutrient inputs and outputs from a watershed. A watershed is an area of land that drains into a single stream or river, as illustrated in Figure 21.11. In a watershed, scientists can estimate the rate of weathering by measuring the net movement of several highly soluble nutrients, such as calcium (Ca2+), potassium (K+), sodium (Na+), and magnesium (Mg2+). These nutrients easily leach out of the soil and move into streams where their concentrations can be measured as the stream leaves the watershed.

Figure 21.11 A watershed. A watershed is an area of land that drains down into a single stream or river.

Watershed An area of land that drains into a single stream or river.

An example of this approach was reported in 2012 for 21 small watersheds in the Canadian province of Quebec. Each watershed contained a small lake with a stream flowing out of the lake that drained the watershed. The watersheds were all forested and had little human activity, so the researchers assumed that nutrient movement was at equilibrium. Researchers monitored inputs of calcium, potassium, sodium, and magnesium entering each watershed through precipitation. In addition, they determined the amount of each element present in the soil and bedrock. Finally, the researchers monitored the amount of each element coming out of the watershed in the stream that drained it. By knowing the inputs and outputs of a watershed, scientists were able to determine its weathering. When they placed their data on a map of the study area, shown in Figure 21.12, they also found that weathering rates varied geographically. The rate of weathering was highest in the southwest region of the province, probably because of regional differences in temperatures, precipitation, and soil conditions.

Figure 21.12 Weathering rates in 21 Canadian watersheds in Quebec. When researchers quantified the combined inputs and outputs of calcium, magnesium, potassium, and sodium, they found that the rates of weathering differed a great deal across the landscape.
After D. Houle et al., Soil weathering rates in 21 catchments of the Canadian Shield, Hydrology and Earth System Sciences 16 (2012): 685–607.

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The Breakdown of Organic Matter

Although weathering of inorganic nutrients provides nutrients to terrestrial ecosystems, weathering is a very slow process. Therefore, primary production largely depends on rapid regeneration of nutrients from detritus. To understand how nutrients are regenerated in these systems, we need to examine the process of decomposition. As we discussed in Chapter 1, scavengers consume dead animals and detritivores break up organic matter into smaller particles. However, decomposition is the process that breaks down organic matter into smaller and simpler chemical compounds and is conducted primarily by bacteria and fungi.

As illustrated in Figure 21.13, the breakdown of plant matter in a forest occurs in four ways: soluble minerals and small organic compounds leach out of organic matter, large detritivores consume organic matter, fungi break down the woody components and other carbohydrates in leaves, and bacteria decompose almost everything. Leaching removes 10 to 30 percent of soluble substances from organic matter, which includes most salts, sugars, and amino acids. What remains behind are the complex carbohydrates, such as cellulose, and other large organic compounds, such as proteins and lignin. Lignin determines the toughness of leaves and gives wood many of its structural qualities. The lignin content of plants is a particularly important determinant of decomposition rate because it resists decomposition more than cellulose. Some portion of the lignins, as well as other plant compounds that resist decomposition, may never be broken down in the surface soils but can form fossil fuels when buried for millions of years.

Figure 21.13 Decomposition of organic matter. organic matter decomposes through the leaching action of water, consumption by invertebrates, mineralization by fungi, and mineralization by bacteria.

Large detritivores, including millipedes, earthworms, and wood lice, also play an important role in decomposition. These animals can consume 30 to 45 percent of the energy available in leaf litter, but they consume a much lower fraction of the energy available in wood. The importance of large detritivores is twofold; they decompose organic matter directly and they macerate organic matter into smaller pieces of detritus. By breaking the organic matter into smaller pieces, the detritivores cause it to have a greater surface area to volume ratio. This gives bacteria and fungi more surfaces on which to act and increases the rate of decomposition.

Bacteria and fungi play an important role in decomposition because they help convert organic matter into inorganic nutrients. Fungi play a special role because the hyphae of fungi can penetrate the tissues of leaves and wood that large detritivores and bacteria cannot penetrate on their own. If you have ever walked through a forest, you may have seen the fruiting bodies of many different fungi, including the impressive shelf fungi that emerge from the sides of dead logs (Figure 21.14). Some fungi and bacteria can break down cellulose and lignin, which are two major components of organic matter that few other organisms can decompose. They secrete enzymes that break down the plant matter into simple sugars and amino acids that they then absorb.

Figure 21.14 Fungi decomposing a dead log. Fungi play a key role in the decomposition of organic matter in terrestrial ecosystems by penetrating the dead tissues of plants. The fruiting bodies of this oyster bracket fungus (Pleurotus ostreatus), commonly referred to as mushrooms, are emerging from a dead log in Belgium.
Photo by Philippe Clement/naturepl.com.

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In terrestrial ecosystems, 90 percent of all plant matter produced in a given year is not consumed directly by herbivores but is ultimately decomposed. Many plants resorb some of the nutrients from their leaves before the leaves are dropped. The aboveground dead plant biomass, combined with the organic matter of dead animals and animal waste, drops onto the soil surface where nutrients are leached. Detritivores break the organic matter into smaller pieces and decomposers break it down even further. Here decomposition is primarily aerobic, and plant roots and their associated mycorrhizal fungi have ready access to the nutrients that are released by the decomposers.

Because plant growth and decomposition are biochemical processes, nutrient cycling between producers and decomposers in terrestrial ecosystems is influenced by temperature, pH, and moisture. The rate of decomposition is also affected by the ratio of carbon and nitrogen in the organic matter. As we discussed in Chapter 20, differences in the stoichiometry of an organism’s food can affect the consumption of the food and the number of consumers that can be supported by the food supply. In the case of decomposition, if the decomposers require high amounts of nitrogen, then low nitrogen availability in the organic matter can cause slower rates of decomposition.

A study of leaf decomposition in Costa Rica provides insights into how several factors affect the rate of decomposition. In this study, researchers gathered leaves recently shed from 11 species of tropical trees and placed them into coarse-mesh bags. The bags were positioned on the ground in the forest where invertebrates had access to the leaves while the leaves remained in the bag. These invertebrates shredded the leaves into smaller pieces with an increased surface area, which facilitated more rapid decomposition. The researchers weighed the leaf bags at different times over a 230-day period to determine the rate of leaf decomposition. You can view the data for several of these species in Figure 21.15a. Leaf species with curves that decline faster over time, such as the Brazilian firetree (Schizolobium parahyba), have the highest rate of decomposition. Such species have relatively low amounts of lignin and cellulose and they have high leaf solubility, which allows more nutrients to be leached out of the leaf. The daily rate of mass loss, which we denote as k, can be calculated for each curve. Species that decompose faster have a higher value of k. In “Analyzing Ecology: Calculating Decomposition Rates of Leaves,” we discuss these calculations in more detail.

Figure 21.15 Decomposition rates of leaves in a tropical forest. Researchers collected recently fallen leaves and placed them into mesh bags to determine the rate of daily mass loss over time in the forest. (a) A sample of 4 of the 11 species examined reveals a range of different decomposition rates. Error bars are standard errors. (b) During the first 50 days of decomposition, the decomposition rates of the 11 leaf species were positively correlated with the solubility of compounds that could be leached from each species. (c) After 50 days, the decomposition rates of the 11 leaf species were considerably lower and negatively correlated to the amount of lignin in each leaf species relative to the amount of nitrogen.
Data from W. R. Wieder et al., Controls over leaf litter decomposition in wet tropical forests, Ecology 90 (2009): 3333–3341.

To examine how leaf traits affected decomposition rate, measured again as the daily rate of mass loss (k), the researchers focused on distinct time periods: from day 0 to 50 when most decomposition would occur due to leaching, and from day 51 to day 230 when most decomposition would be due to invertebrates, fungi, and bacteria. During the first 50 days, there was a positive relationship between the decomposition rate and the fraction of soluble compounds in the different leaf species that can be leached, as shown in Figure 21.15b. During the latter portion of the experiment, there was a negative relationship between decomposition rate and the ratio of lignin to nitrogen, as you can see in Figure 21.15c. This means that leaf species with a relatively high amount of lignin experience a slower rate of decomposition. From this study we see that the decomposition of organic matter depends on both the environmental conditions and the chemical traits of the organic matter.

Once researchers calculated values of k for each species, they could examine how both precipitation and the traits of the leaves affect the leaf decomposition rate. For example, they manipulated the amount of precipitation that some of the mesh bags received. When they reduced precipitation to 50 or 25 percent of normal levels, it caused a 10 to 20 percent decline in the rate of decomposition.

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Decomposition Rates Among Terrestrial Ecosystems

Because environmental conditions are a key determinant for rates of decomposition, terrestrial ecosystems differ a great deal in their decomposition rates. Comparative studies of temperate and tropical forests show that detritus in the tropics decomposes more rapidly because of warmer temperatures and higher amounts of precipitation. For example, we can compare the amount of dead plant matter on the forest floor—including leaves, branches, and logs—versus the total biomass of vegetation and detritus in a forest. The proportion of dead plant matter is about 20 percent in temperate coniferous forests, 5 percent in temperate hardwood forests, and only 1 to 2 percent in tropical rainforests. Of the total organic carbon in terrestrial ecosystems, more than 50 percent occurs in soil and litter in northern forests, but less than 25 percent occurs in tropical rain forests where the majority of the organic matter exists in the living biomass.

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ANALYZING ECOLOGY

Calculating Decomposition Rates of Leaves

Researchers often want to examine the rate of leaf litter decomposition to determine how fast nutrients can be regenerated so they are available to producers, such as algae that form the base of an aquatic food web, plants that form the basis of a terrestrial food web, or last year’s crops that decompose and provide nutrients to the current year’s crop. As we have seen, when researchers want to estimate decomposition rates of leaves to determine how rapidly nutrients are regenerated in an ecosystem, they commonly weigh samples and put them into mesh bags that are positioned on the ground where they will decompose. Over time, the researchers collect, dry, and weigh the bags to determine how much mass remains and how much has been decomposed. The rate of decomposition commonly follows a negative exponential curve; there is initially a rapid loss in mass that slows over time. This negative exponential curve can be described by the following equation:

mt = mo e−kt

where mt is the mass of leaf litter that remains at a particular time, mo is the original mass of leaf litter, e is the base of the natural log, k is the daily rate of mass loss, and t is time, which is measured in days. The decay constant k is the key parameter in this equation because it determines the shape of the curve; leaves that decompose at a faster rate have a larger value of k.

Ecologists can estimate the value of k in this equation with statistical software that determines the line that best fits a set of data for decomposition over time. Once the statistical software determines the values for the daily decomposition rate, we can then estimate the amount of leaf litter at any point in time provided that the decomposition occurs under similar environmental conditions. For example, if we start with 100 g of leaves and k = 0.01, we can estimate the mass of leaves that has not decomposed after 10, 50, and 100 days: After 10 days:

After 10 days: mt = mo ekt = 100 e−(0.01)(10) = 90 g

After 50 days: mt = mo ekt = 100 e−(0.01)(50) = 61 g

After 100 days: mt = mo ekt = 100 e−(0.01)(100) = 37 g

These differences in litter decomposition rates mean that tropical forests have a much larger proportion of the total organic matter in living vegetation than in detritus. This has important implications for tropical agriculture and conservation. For example, when tropical forests are cleared and burned, a large fraction of the nutrients are mineralized by burning and by subsequent high rates of decomposition. Together, these processes create an abundance of nutrients during the first 2 to 3 years of crop growth, but any surplus nutrients not taken up by the crops quickly leach away. Traditionally, tropical areas burned for agricultural fields would be farmed for 2 to 3 years and then left to undergo natural succession for 50 to 100 years to rebuild the fertility of the soil. In modern times, however, many regions have human populations that are too dense to allow rotation of agriculture into different areas over several decades. Without rotation, the soils cannot replenish their nutrients and the fertility of the land rapidly degrades.