Gains from Wood et al. (2009) – Rain forest nutrient cycling and productivity in response to large-scale litter manipulation

Shan Xu

There exists time lag between litter production and the release of nutrient from litter. Seasonal variability, forest age and soil fertility influence both litter production and the release of nutrient (or decomposition rates). Therefore, it is difficult to quantify the linkages between litter production, nutrient availability and net primary production. Nutrient availability from litter, which affects net primary production, depends on litter production, decomposition rates and nutrient loss. Thus Wood et al. (2009) conducted a large-scale litter manipulation experiment in forests with different ages and varying soil fertility in a wet tropical forest in Costa Rica to explore these linkages.

    The experiment design is six locations, three treatments and two replicates. The details are listed below:

Six locations:

L3: lower-fertility & older oxisols

L5: lower-fertility & older oxisols

A2: lower-fertility & younger oxisols

A4: higher-fertility & younger oxisols

LP: low fertility & secondary forest & older oxisols

LS: high-fertility & secondary forest & older oxisols

Three treatments:

Control

Litter addition

Litter removal

Two replicates

Measured variables: leaf litter mass, leaf litter N and P concentrations, and leaf litter N and P inputs, forest floor turnover time and decomposition rates of litter

Factors driving the response of production or inputs to litter manipulation:

quantity of litter added/removed, quality of litter added/removed ( [N] & [P] ), quantity of nutrients in the added litter/removed, and soil nutrients (total N and P, Bray-1 P);

the stoichiometry: C:N, C:P, N:P;

forest floor turnover time, basal area, and stem density;

The main results are listed below:

1)      Litter removal had no effects on forest productivity or nutrient cycling while litter addition significantly increased leaf litter production and N and P inputs. However, litter addition had no significant effect on woody growth.

2)      As much as 41- 62% of the variation in the cumulative leaf litter, litter N, and litter P inputs of the addition plots was explained by the total P in added litter.

3)      Soil fertility influences litter quality (litter [N] & [P]), but not litter quantity (litter production).

4)      Litter [N] and litter [P] responded differently to soil fertility and forest ages, with that litter [P] was significantly higher in the high-fertility sites while litter [N] was significantly higher in the secondary forests than in the old-growth forest plots.

5)      Soil fertility influenced forest floor turnover time, but not decomposition rates of litter.

 

In the discussion parts, they discussed the forest floor turnover rate and the main factors that could explain the increased forest productivity under litter addition. They suggested soil moisture, pH, and temperature were generally unchanged to litter manipulation and total P added was the main driver of that variability. And they also suggested soil fertility had indirect effect on leaf litter production via its influence on litter quality. While litter removal had no significant effect on forest productivity and more intense litter removal over a longer time period is likely needed. Over such a short time, litter addition did not increase wood production and further research is needed.

     About litter nutrient cycling, they suggested the short-term variation in leaf litter nutrient concentrations is not derived from the sudden large availability of nutrients and the effect of litter addition on both N and P inputs was short-lived. In addition, across sites, the maximal effect of litter addition occurred in October-November of the year of addition, regardless of litter quality. They attributed this to the first response of microbes to climate and then to litter quality.   

     In tropical forest, hurricanes and major storm events, or dry year will lead to massive defoliation, which resulted in a net positive feedback on leaf litter production and nutrient inputs to soil. However, the efficient nutrient recapture may enable tropical trees to adapt to those climate events.

 

Some enlightenment gained from this study:

Given that litter is one of the largest nutrient fluxes from plants to soil, it is generally accepted that litter addition would increase litter N and P inputs to soil while litter removal would decrease them. However, the results of this study showed litter removal did not decrease litter N and P inputs to soil. They authors attributed to the less intense litter removal treatment and short time-scale. Therefore, it could be considered that the initial soil fertility can supply soil nutrients in short term and the decomposition of nutrients from soil organic matter would be accelerated. However, in the results of our meta-analysis, litter addition had no significant effects on both soil N and P while litter removal generally decreased them. This may be explained by the initial less fertile soil and the nutrients recaptured from litter is the major nutrient sources to soil.  Further to study soil nutrient cycling under litter manipulation is necessary and significant to explore the below-ground dynamics, which are still unclear and urgently need to be clarified.

 

Some excellent sentences from this study:

1)      That the majority of nutrients mineralized from these inputs are then recaptured and feed back positively to net primary productivity and nutrient cycling is an important paradigm of biogeochemical cycling in terrestrial ecosystems.

2)      The long lag between the timing of litter production and the subsequent release of those nutrients from the decomposed litter makes it difficult to quantify these linkages due to the difficulty in distinguishing effects of litterfall from those of other factors known to influence forest productivity, such as forest age, soil fertility, and seasonal changes in climate.

3)      Differences in litter quality and in decomposition rates might lead to differences among forests in the timing and the magnitude of their response to litter inputs.

4)      We predicted that secondary forests would exhibit the largest and earliest response to litter addition while old-growth forests located on low-fertility soils would display the smallest and most delayed response to addition.

5)      The similar timing of this response, indicates that microbes are responding to climate first, and to litter quality second.

6)      Variability in the response of vegetation to litter addition was driven by variability in the total amount of the most limiting nutrient (P) in added litter rather than total organic matter inputs to the soil or soil fertility, and these nutrients were put toward new leaf production rather than wood growth.

 

 

                                                         Feb.6th, 2012

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