Nutritional Contribution of Litter in Rainforest of Brazil

Lowlands Dense Ombrophilous Forest is one phytophysiognomies of Atlantic Forest in Brazil. The main ecological characteristic of this forest is the Ombrophilous environment, related to high rainfall and temperature indexes. Nutrient cycling is well balanced in the periods of good thermo-pluviometric distribution. Global climatic changes have been intensifying in recent years making rainfall irregular, changing its distribution and intensity throughout the year. This can affect the natural regeneration and vegetative growth of the species. This study aimed to correlate litterfall and nutrient contribution with climatic variations, identifying the level of importance of this correlation and which nutrients may have their compromised cycling. Literfall was collected monthly in 40 collectors. N, P, K, Ca and Mg contents were determined and their stocks were calculated. The litter deposition was 8,261.15 kg ha year and was not influenced by rainfall and temperature. The N, P, K, Ca and Mg stock in this litter was 244.93 kg ha year, being stored just of N 113.75 kg ha year. P and K stocks varied with rainfall and temperature, suggesting that variations in these climatic variables may interfere in the cycling of these nutrients in this forest fragment.


Introduction
Nutrition of tropical forests is supplied by the stock of nutrients transferred to the soil.The contribution of litter is the main responsible for the entry of nutrients into the forest system (Wood, Lawrence, Clark, & Chazdon, 2009;Diniz, Machado, Pereira, Balieiro, & Menezes, 2015).
Litter also acts in light interception, shading of seeds and seedlings, and reducing water evaporation.Additionally, it aids in diminishing the impact of raindrops on soil, which reduces surface runoff and nutrient loss (Li, Niu, & Xie, 2014).It is a source of C and energy for soil organisms, and also considered the most dynamic and probably the most variable fraction not only between ecosystems but also within the same ecosystem (Jacoby, Peukert, Succurro, Koprivova, & Kopriva, 2017).
The formation of litter layer depends on production and decomposition rate of organic matter that varies according to the substrate composition, decomposers activity, environmental conditions, especially temperature and relative humidity, and physical properties of the soil (Xiaogai et al., 2013).
The contribution of each vegetal material varies depending on the vegetation typology and climatic condition (Banegas, Albanesi, Pedraza, & Santos, 2015).One way to evaluate this variation is monitoring litter production.Thus, it is possible to comprehend the process of nutrient cycling, to evaluate the productive capacity of the forest, and to relate available nutrients to the nutritional needs of the species.In fact, litter transport is an important bioindicator of climatic variations in forest environments, because it may vary according to air temperature and/or rainfall (Llausás & Nogué, 2012;Ferreira, Silva, Pereira, & Lamano-Ferreira, 2014).
The largest litter contribution is due to water deficit in soil, which usually occurs during dry seasons (Silva, Poggiane, Lima, & Libardi, 2014).When this phenomenon is combined with high air temperatures, forest species are subjected to water stress and loss of plant material (mainly leaves), increasing the supply of litter on the soil.Additionally, since there is little moisture in soil during this period, the decomposition is reduced and nutrients a higher air rainfall.

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Study A
The study Pernambuc 263220, 2 (Figure 1 Exchangeable Ca 2+ , Mg 2+ and Al 3+ were extracted with 1.0 mol L -1 KCl and determined by titration.P, K + , Fe, Cu, Zn and Mn were extracted by Mehlich-1.Available P was determined by spectrophotometry, K + by flame photometry, and Fe, Cu, Zn and Mn by atomic absorption spectrophotometry.Potential acidity (H+Al) was extracted with 0.5 mol L -1 calcium acetate and determined by titration.With these results, sum of bases (SB), base saturation (V), Al saturation (m), effective cation exchange capacity (CEC effective ) and potential cation exchange capacity (CEC potential ) were all calculated.Physically, the soil was characterized by particle-size distribution, defining its textural class.All analytical procedures were according to P. Teixeira, Donagema, Fondana, and W. Teixeira (2017).

Collection of Litter Supply and Analytical Procedures for Nutrients Determination
Next to each 40 sampling sites used for chemical and physical analysis of the soil, a collector made by nylon net with one-millimeter mesh and 0.25 m height walls was installed, in a suspended form at approximately 0.50 m above the ground.The litter supply was evaluated monthly from June 2014 to May 2015.
Each month, the deposited litter was packed in labeled paper bags and brought to a forced air circulation chamber at 65 o C until a constant weight was reached.This material was weighed for determination of dry weight, milled, homogenized and packed in previously cleaned and dried vials.

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Litter D
The annua observed i Atlantic Fo more adva changes oc Curtis, Ha rainfall, and was highest in the dry season, when evaluating the relationship between litter production in primary and secondary forests in the Brazilian Amazon.Espig et al. (2009) found a negative correlation between litter production and rainfall in an Atlantic Forest fragment in Pernambuco, Brazil.The correlation coefficient was low when the authors considered the entire litter deposited [r = -0.5491(p < 0.05)] and a little higher [r = -0.5853(p < 0.05)], when they considered only leaf deposition.
Rainfall variations throughout the year are very frequent in tropical regions, which can influence the contribution of litter in an inversely proportional way.However, this did not occur because rainfall from June to September 2014 and from March to May 2015 were very variable (Figures 2 and 4), preventing a significant correlation with litter supply.
The air average temperature, especially near oceans, does not vary much (Figure 2) because there is a thermal regulation promoted by the relative humidity of the air coming from the ocean, which makes the contribution of litter not have correlation with this climatic variable (Figure 4).

Litter Nutrient Contents
Nutrient contents provided in the litter corresponded to the following decreasing order: N > Ca > Mg > K > P (Table 2).Dickow, Marques, and C. Pinto (2009) also observed this behavior in an area with tree species from secondary rainforests in the South of Brazil.Note. 1 Coefficient of variation = 100 × standard deviation/average.Averages followed by equal letters in columns do not differ from each other by Scott-Knott test (P < 0.05).**Significant by test F (P < 0.01).
Villa, Pereira, Alonso, Beutler, and Leles (2016) observed that P had the lowest concentration (N > Ca > K > Mg > P) in restoration areas.Pinto, Martins, Barros, and Dias (2009) also observed this same distribution sequence in a semideciduous seasonal forest at the beginning of succession in Viçosa, Minas Gerais, Brazil.
N contents ranged from 12.59 to 14.94 g kg -1 , with no significant differences between the months July, August and December 2014, and March, April and May 2015 (higher levels), as well as between June, September, October and November 2014, and lastly January and February 2015 (lower levels) (Table 2).
The P average content recorded was 0.95 g kg -1 (Table 2).This value is higher than that determined by Espig et al. (2009), which was 0.50 g kg -1 .The authors reported that the fragment presented a more advanced successional stage, which caused P to be translocated from old leaves to young ones due their mobility.
The K average content was 1.73 g kg -1 (Table 2).K content in litter observed in this study was lower than that determined by Espig et al. (2009) in Atlantic Forest area of Pernambuco, Brazil.This may be related to low rates of this nutrient in biogeochemical cycling (Smith et al., 2015).The low K content may also be related to the ease of this nutrient leaching directly from leaf surface by rainwater, due to its high solubility.
Ca presented a mean content of 7.93 g kg -1 (Table 2).This high content occurs because Ca is a structural component found in the cells of the plant tissue, and one of the last nutrients to be released to the soil through litter decomposition (Dickow et al., 2009;Villa et al., 2016).However, this may also be related to the low mobility of Ca in plant tissues and to the longevity of leaves, which causes it to remain in larger amounts in senescent leaves than other nutrients (Maillard et al., 2015).
Mg contents presented significant differences between the evaluated months (Table 2).The average content was 5.50 g kg -1 , higher than those determined by Golley et al. (1978) and Espig et al. (2009), which were 2.04 g kg -1 and 2.42 g kg -1 , respectively.Low content can sometimes be explained by the translocation of this nutrient from senescent leaves to younger ones, decreasing the concentration in the litter (Maillard et al., 2015).The soil of this fragment studied presented high levels of exchangeable Mg (Table 1), reflecting on nutrition of species and consequently on contribution.

Nutrients Supply of Litter
The annual nutrient contribution from litter to soil presented the following decreasing order: N > Ca > Mg > K > P (Table 3).Contribution of N and Ca is high in most of the analyzed tropical forests.This happens especially because both occur in larger proportions in leaf component (Lima et al., 2018), which corresponds to most of the litter (Kumar & Tewari, 2014).
Table 3. Monthly and annual contribution of nutrients from litter in the fragment of Lowlands Dense Ombrophilous Forest, Brazil Note. 1 Coefficient of variation = 100 × standard deviation/average.Averages followed by equal letters in columns do not differ from each other by Scott-Knott test (P < 0.05).** Significant by test F (P < 0.01).
The amount of nutrients contributed to soil was 244.93 kg ha -1 year -1 (Table 3).N was the most contributed (46.4% of annual contribution) and P the least contributed (3.2% of annual contribution).Forest nutrition may be dependent on nutrient cycling because the soil of the fragment presented low natural fertility (Table 1).The nutrient cycling may be influenced by climatic variations, both for nutrient supply and for decomposition of the vegetal material.
Aerts (1996) explained that P is a very mobile element in plant.The translocation occurs from 40 to 60% of this element located in the older leaves to the younger organs of the plant before foliar abscission.This causes this

Table 2 .
Monthly concentrations and annual average of litter nutrients in the fragment of Lowlands Dense Ombrophilous Forest, Brazil