Importance of nutrient cycling in relation to inputs and outputs

Because many nutrient losses from terrestrial communities are channeled through streams, a comparison of the chemistry of streamwater with that of incoming precipitation can reveal a lot about the differential uptake and cycling of chemical elements by the terrestrial biota. Just how important is nutrient cycling in relation to the through-put of nutrients? Is the amount of nutrients cycled per year small or large in comparison with external supplies and losses? The most thorough study of this question has been carried out by Likens and his associates in the Hubbard Brook Experimental Forest, an area of temperate deciduous forest drained by small streams in the White Mountains of New Hampshire, USA. The catchment area - the extent of terrestrial environment drained by a particular stream - was taken as the unit of study because of the role that streams play in nutrient export. Six small catchments were defined and their outflows were monitored. A

Table 18.1 Annual nutrient budgets for forested catchments at Hubbard Brook (kg ha-1 year-1). Inputs are for dissolved materials in precipitation or as dryfall. Outputs are losses in streamwater as dissolved material plus particulate organic matter. (After Likens et al., 1971.)

NH+

NO3

K+

Ca2+

Mg2+

Na+

Input

2.7

16.3

1.1

2.6

0.7

1.5

Output

0.4

8.7

1.7

11.8

2.9

6.9

Net change*

+2.3

+7.6

-0.6

-9.2

-2.2

-5.4

* Net change is positive when the catchment gains matter and negative when it loses it.

network of precipitation gauges recorded the incoming amounts of rain, sleet and snow. Chemical analyses of precipitation and streamwater made it possible to calculate the amounts of various nutrients entering and leaving the system, and these are shown in Table 18.1. A similar pattern is found each year. In most cases, the output of chemical nutrients in stream flow is greater than their input from rain, sleet and snow. The source of the excess chemicals is parent rock and soil, which are weathered and leached at a rate of about 70 g m-2 year-1.

In almost every case, the inputs and outputs of nutrients are small in comparison with the amounts held in biomass and recycled within the system. Nitrogen, for example, was added to the system not only in precipitation

... whereas a young forest is a net carbon source (output greater than input)

the movement of water links terrestrial and aquatic communities

Hubbard Brook -forest inputs and outputs are small compared to internal cycling

(6.5 kg ha-1 year-1) but also through atmospheric nitrogen fixation by microorganisms (14 kg ha-1 year-1). (Note that denitrification by other microorganisms, releasing nitrogen to the atmosphere, will also have been occurring but was not measured.) The export in streams of only 4 kg ha-1 year-1 emphasizes how securely nitrogen is held and cycled within the forest biomass. Stream output represents only 0.1% of the total nitrogen standing crop held in living and dead forest organic matter. Nitrogen was unusual in that its net loss in stream runoff was less than its input in precipitation, reflecting the complexity of inputs and outputs and the efficiency of its cycling. However, despite the net loss to the forest of other nutrients, their export was still low in relation to the amounts bound in biomass. In other words, relatively efficient recycling is the norm.

In a large-scale experiment, all the trees were felled in one of the Hubbard Brook catchments and herbicides were applied to prevent regrowth. The overall export of dissolved inorganic nutrients from the disturbed catchment then rose to 13 times the normal rate (Figure 18.6). Two phenomena were responsible. First, the enormous reduction in transpiring surfaces (leaves) led to 40% more precipitation passing through the groundwater to be discharged to the streams, and this increased outflow caused greater rates of leaching of chemicals and weathering of rock and soil. Second, and more significantly, deforestation effectively broke the within-system nutrient cycling by uncoupling the decomposition process from the plant uptake process. In the absence of nutrient uptake in the spring, when the deciduous trees would have started production, the inorganic nutrients released by decomposer activity were available to be leached in the drainage water.

The main effect of deforestation was on nitrate-N, emphasizing the normally efficient cycling to which inorganic nitrogen is subject. The output of nitrate in streams increased 60-fold after the disturbance. Other biologically important ions were also leached faster as a result of the uncoupling of nutrient cycling mechanisms (potassium: 14-fold increase; calcium: sevenfold increase; magnesium: fivefold increase). However, the loss of sodium, an element of lower biological significance, showed a much less dramatic change following deforestation (2.5-fold increase). Presumably it is cycled less efficiently in the forest and so uncoupling had less effect.

Figure 18.6 Concentrations of ions in streamwater from the experimentally deforested catchment and a control catchment at Hubbard Brook. The timing of deforestation is indicated by arrows. Note that the 'nitrate' axis has a break in it. (After Likens & Borman, 1975.)

J JASONDJ FMAMJ JASONDJ FMAMJ JASONDJ FMAM

1965 1966 1967 1968

Year

J JASONDJ FMAMJ JASONDJ FMAMJ JASONDJ FMAM

1965 1966 1967 1968

Year deforestation uncouples cycling and leads to a loss of nutrients

Figure 18.6 Concentrations of ions in streamwater from the experimentally deforested catchment and a control catchment at Hubbard Brook. The timing of deforestation is indicated by arrows. Note that the 'nitrate' axis has a break in it. (After Likens & Borman, 1975.)

Was this article helpful?

0 0
Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment