Dear Dr. Srivastava.

Thank you for your questions, all of which raise extremely interesting points. I'll attempt to answer a few of these as follows:

Firstly: rate of litter decomposition is likely influenced by a range of factors, relating to meteorological factors (such as temperature; precipitation and air movement as it influences microclimate at the soil/plant litter/air interface); soil factors (including physical, chemical and mostly biological properties of the soil in question); the physical/chemical nature of the litter itself (perhaps mostly in terms of its content of such complex chemical components as lignification, suberins, waxes and oils: collectively conferring upon the residue what is called the "litter quality); and biotic factors such as previous management practices applied to the soil in question. I've no doubt there are many more factors, but for now, I'll leave it at that.

I fully agree with Dr Eslamikhah's point that the process of litter decomposition is not always beneficial for rhizosphere health, since decomposition of litter under conditions of soil water-logging(and hence, anaerobic conditions) may result in derivatives that do not support healthy growth of roots, and which therefore (through possible effects on root exudation of plants so affected) do not benefit the rhizosphere.

I'd like to comment on your third question (how does rate of litter decomposition vary temporally and spatially?) as follows. I recommend that any pruning debris from such as citrus trees be put through a suitable wood-chipper and scattered on the surface of the soil underneath the drip zone of tree canopies, and that mown grasses and herbs from the orchard inter-rows (I do not recommend use of herbicides to 'control" vegetation growing in this valuable area of the orchard) be strewn on top of the wood chip residues under the trees, and so on every year.

  These residues are NEVER worked into the soil: they are just added to the surface in a series of layers, every year. What is evident is that, depending upon the irrigation system used, the fastest breakdown of litter is always at the soil litter boundary. What we see within a few years of using this practice is that the bottom of the litter layer and the top of the soil layer become very much the same, and become filled with perhaps the healthiest roots that the trees produce. With trees under drip irrigation, the top of the litter layer dries out and seems to remain more or less intact (depending on climatic conditions such as rainfall and temperature). However, where trees are irrigated with micro-sprinklers, which wet an extended area of the soil surface, the entire layer of litter seems to break down more uniformly and also more rapidly.

Your fourth question concerns the extent to which fertiliser applications may be reduced where active decomposition of plant litter is evident. I can only offer an opinion based on my practical experiences, where on mature orange trees (a seedless valencia cultivar called 'MidKnight', we were able to reduce N applications by 50% (from about 250 units N [per ha/per year] ) and K applications by 33% (from about 140 units K) for the same yield (about 58 tonnes/ha), but with fruit under the "reduced N and K treatment: colouring four weeks earlier, showing higher fruit brix % (about 20% higher); lower titratable acidity (which resulted in superior brix/acid ratio), and according to subjective assessments, a deeper orange juice colour and superior flavour. I would advise great caution in your interpretation of this: this was not a scientific trial, merely an on-farm proof of concept field trial, although each 'treatment"  comprised about 3000 trees. What I will say is that my experience suggests that growers fertilise their citrus trees to maintain leaf N levels at about 2.0 to 2.2% N (compared with a "standard" of 2.6 to 2.8%) and K levels at about 08 to 1% (compared with 1.2 to 1.4% recommended). Tissue sampled in this case were leaves picked behind the fruit on a fruit-bearing terminal, which is standard practice in South Africa.

I hope these thought have been of interest to you. With warmest wishes.

Anoop sir,

Besides addition of nutrients to soil, the litter deco,position also releases organic acids which cuases the soil acidity and remove exchangeable cations. Examples are formation of laterites and podzols under hot humid and cold humid conditions, respectively.  Hence, it may not be always beneficial to rhizosphere of soil.

Mostly the nitrogen is added back to soil and the P and K are removed from soil and lost through timber / wood removal.  However, addition of litter also contribute very little amount of P and K. It depends on plant species.

Thanks,

Dr. Anoop Kumar asked very important questions that are of practical nature and these aspects are mostly ignored because the popular notion is "organic matter addition is always useful". I am very happy that Dr. Andrew presented very elaborated, precise and accurate answers to all the questions. His efforts are really appreciable because he was so kind to spare so much time for the benefit of other researchers who are directly or indirectly attached to this type of work. I am sure that this debate will be highly useful and interesting as well.

As other researchers, Rajakumar and Reyhaneh, indicted too that addition of litter to soil may or may not prove useful. This cannot be taken as rule of thumb. It will depend upon the nature of the litter itself, soil characteristics, especially pH, climatic conditions, and soil moisture etc. These factors will not only control the rate of decomposition but also the usefulness of litter, if it is going to be at all under prevailing conditions of the situation. The combination of these factors will also tell the nutrients quantities the litter is going to add. Sometimes, the contaminated litter may harm the soils even.   

Ms Reyhaneh, I agree with you,  litter decomposition is not always beneficial to the upliftment of rhizosphere dynamics from multiple angles. This  was the basic reason to flag off such an issue whose consequences are beyond soil health off course. I can visualise the rhizosphere of Acacia trees , you will hardly see anything growing underneath tree canopy  , but in the bulk soil zone , there are plenty of vegetation , you can easily find.   

Dr Krajewski , your response is so methodical and informative that I feel mesmerised  , its so wholesome answer. What is  your opinion about developing the rate of litter decomposition (k) database for crop specific litter . We have seen in mandarins ,  litters getting nicely decomposed only in the wetting zone of drippers compared to the zone outside the drippers. The extent of litter decomposition is not uniform within the rootzone. How do you see the role of  root exudates from specific crop, the total nutrient composition of which  could further dictate the rate of decomposition , and consequently the composition of microbial diversity within the rhizosphere.  It is an interesting input never plough the litter within the soil , let it decompose on the soil surface .

Dr Rajakumar, I agree with you the organic acids being released , the composition of which again translate the health of the rhizosphere. And soil acidity is an example of root exudate plus the acidity released upon decomposition of pine needles under temperate climate . You hardly see any other vegetation underneath pine trees , simply because of extreme  soil acidity .

Dr Nazir , the role of litters thrown up by the deciduous perennial trees so enormous that very often , knowingly we undermine   its utility in the context of cutting down the nutrient requirement of main crop. The excellent supplement of Dr Krajewski is an eye opener . Hope , he will  further supplement his informative  responses in the discussion in progress.   

Dr. Anoop Kumar! Your response to different answers is highly logical. This is really a new side of the application of litter. Let us receive some good information from other scientists. I am looking that this discussion is going to be eye opening and deleting or misunderstanding of old concepts. 

Thank you Dr Nazir for opening up the discussion so nicely  an dso effectively. And  encouraging our learned colleagues to express their heartout experiences on such an evasive issue.

I am not sure about water retarders being build up within the rhizosphere as a result of decomposition of  leaves carrying waxy surfaces , but this possibility , you cannot outrightly deny . We have in perennial fruit crops , especially citrons , lemons , pomelos , the plants growth after reaching to some peak , start ceasing automatically . Is it sehlf degenerating  habit ?. , remains to be investigated . We have observed in old declining citrus trees , initially having a good drainage, and all, but same trees after 30 years or so , start showing declining symptoms due to battery of issues , but low hydraulic conductivity  coupled with poor water transmission properties is predominantly one of those causal factors. Despite all these , water availability surely controls the litter  decomposition rate and minerlization rate   having strong impetus on the N and C annual flux. Enclosed below  a PDF will  throw more lights on litter decomposition  rate vis-a-vis water availability.  

While extending the discussion further , which I feel is still incomplete . Lets take the rate of litter decomposition which will dictate the rate of nutrient release as a result of mineralization . the composition of litter plays a pivotal role with regard to extent of decomposition . For example , decomposition of beech litter, considering the loss in mass as one of the most reliable criteria of rate of litter decomposition , as much as 17-81% of litter mass was decomposed after two years . Beech roots were least susceptible to decomposition , while  spruce needles were most decomposable. However , the rate of litter decomposition was observed highly dependent upon  content of water soluble substances.Find below some related PDFs for further reading .

This is a very good question . I am really interested to from you Dr Srivastava and other colleagues , what conditions of litter decomposition really govern the dominance of one microbial species over other . And what is the best technique of studying the litter decomposition rate .

Dr Singh , with regard to conditions triggering the litter decomposition could be elaborated as  climatic conditions in terms of diurnal variation in temperature , rainfall  ;nature and properties of soil including texture , aggregate size distribution, organic carbon , acidity , salinity/sodicity, microbial build-up etc. chemical composition of litter including amount of lignin,  linin to protein ratio , celluloses , C:N ratio etc.  Litter bag incubation method is considered  still the best method to measure the rate of litter decomposition.

Thank you Dr Srivastava for excellent response. I am willing to know the criteria to be used to define rhizosphere health. How different criteria respond to different management inputs.

Dr Singh , if you talk of   criteria of rhizosphere properties  dictating the rhizosphere health , I will straightway enlist the microbial population  and microbial biomass nutrients. These are considered as mast sensitive indicators of early responses in rhizosphere properties  .

Friends , I want to draw the attention of all of you to the very old work of Prof Veron C. Jamison  who for the first time observed the phenomenon of water repellency in soils of central Florida growing Valencia orange . He observed this water repellency most pronounced under the declining trees located in the spreading boundary between declining and healthy productive trees. He alter established the relationship between soil wettability and spread of decline in citrus in central Florida in 1950s. Another interesting observations , he made  by studying the repellency of soils receiving various fertility treatments , which showed wetting resistance was higher in those treatments receiving magnesium or lime. Such water repellency phenomenon , he observed in crops like oak , wiregrass cover . Later , it was concluded that accumulation of bitumens or other products of plant tissue decay could be responsible for phenomenon ( Exerpts from one paper (  Title : Wetting resistance under citrus trees in Florida) presented in a symposium in 1968) . I just wanted to share this information , with regard to litter decomposition . If any one of you have observed similar phenomenon , we will be grateful to you to share such  information .

Interesting Abhishek  and thank for your response. I wonder , how come you get such an impressive collection  on such varied issues. None the less , i am so appreciative of you . Is there any publication on , how type of litter decomposition facilitates or shortens the microbial diversity within the plant rhizosphere?. Looking forward to your impressive response..

My profound thanks for being so prompt and very pertinent links plus PDF . Let me first go through these to respond to you later. 

Thanks Dr Anil Singh and Pierlorenzo for adding some interesting responses. It will be interesting to see , how the structural and functional diversity of of pre-dominant microbial communities undergo reorientation in response to composition of litter undergoing decomposition. 

litter decomposition in soil depends on both biotic (microbial) and abiotic (climate) factors. The importance of these factors in controlling litter decomposition may vary with spatial scale. It mainly depends on the amount of litter and place where it is dumped. It effects the nutrients availability in soil.

if we take this discussion on the other hand in which I may deviate from your question prof. now a days, much research work is carried on production of biogas. If the same litter is dumped in a specific place than it can help in production of natural gas and also a manure for soil.

2ndly I have seen some news regarding the plastic degradation fungi. if a research work carried out in this field so it will help better soil rhizosphere.

Abhshek , some outstanding responses , full of knowledge oriented fedbacks, woth reading all of them. Determining rate of litter decomposition is most important. 

Litter bags with different mesh sizes have been used to manipulate

the composition of these communities (Crossley and Hogland, 1962). The rate litter decomposition was determined by loss in weight. Litter decomposition (i.e. monthly weight loss) was determined from the difference between the weights remaining in the litter bags in each month. To calculate weight loss over time, negative exponential decay model (Olsen, 1963) was used and decay model are given below:

L/Lo= exp (- kt),where, L is the weight remaining at time t, L0 the initial weight, exp the base of natural logarithm, k the decay rate coefficient and t the time (year). The required time for 50% (t50) and 99% (t99) decay was calculated as t50= 0.693/ k and t99= 5/k. While absolute amount of nutrient in the litter bag was investigated as (C/C0) x ( L/L0 ) x 100, where, C is the nutrient concentration in the litter samples at the time of sampling, C0 is the nutrient concentration of the initial litter. L is the mass of dry matter at the sampling and L0 is the initial dry mass of the litter sample. 

Effect of climate and litter quality on decomposition is anothe rvery importatnt component to define the imapct of litter decomposition of soil changes. Meentemeyers proposed a general decomposition model

driven by actual evapotranspiration (AET) as a climatic variable and lignin concentration as a litter quality variable. This model has been used on a reional scale. For some types of litter, the climate is clearly more important  at least in early decomposition stages-than substrate quality as a predictor of decomposition rates over large scales. Meentemeyer and Bergs found that AET was a good predictor of mass-loss rates for Scats pine needles along a Scandinavian climatic transect . Nevertheless, in the same transect, no climatic factor was significantly correlated with mass loss rates of Norway spruce needles. Also, in the same climatic transect, decomposition rates of highly decomposed than on  climate. Furthermore, in a recent study using 39 coniferous forest sites spanning an area from the subarctic to subtropics, including Atlantic and Mediterranean climates, It has also been  also shown AET to be the leading constraint on the mass loss of Scats pine needle litter during the first year of decomposition (accumulated mass loss ~50%). However, other climatic factors, such as seasonal distributions of  precipitation and temperature, explain part of the variation,  and it is possible to distinguish between the effects of inland  and Atlantic climates  using AET as a predictor.

During litter decomposition, the relative lignin concentration apparently increases owing to its low degradation rate”, but also because the conventional lignin analytical methods do not distinguish between true lignin and partially humified products. However, the rate-retarding influence of lignin on litter decomposition appears to depend on climate. Thus, it was found that in late decomposition stages the effect of higher lignin concentrations was low in a harsh climate (e.g. at the Arctic Circle), whereas when the same substrate was decomposed in warmer and wetter regions (European-Atlantic climate), higher lignin concentrations had a predominant effect. The chemical composition of a given litter type, at a given stage of decomposition, depends on the local climatic conditions: at the same level of mass loss the concentration of lignin and lignin-like products is higher at warmer and wetter sites where the initial decomposition rates are high . This could possibly be explained by climatically controlled modifications of the structure of the microbial population and by the humification process, None the less, in these late stages of decomposition, the effect of climate is dominated by the increase of lignin concentration. During these stages, there are strong indications that naturally high levels of N have a rate-retarding influence on the litter decomposition rates( Based on Review of Couteaux et al.1995, published in TREE).

Some important thoughts ......

Thanks Abhishek for so lively discussion on the isssue , and rejuvenating the whole discussion from newer angles. Hydrological and cryospheric systems

will be affected by shifts in precipitation regimes and soil warming, thus altering snowline, soil moisture, soil disturbance regime and ultimately ecosystem

distribution and function. Plant tissues will undergo various physiological changes in response to elevated CO2 concentrations and altered temperature regimes. Overall, organisms close to the limits of their physiological tolerances may be particularly responsive to environmental change. Most vulnerable will be ecosystems lying around the snowmelt isotherm,where for every degree increase in temperature ,the snowline will rise by about 150 m t.  Guvazov (2015, Pl.Soil , 337:19-32)  in his review addressed to alpine environment  highlighted following objectives:

 (1) Biological plant litter decomposition does not exhibit a stereotyped

kinetically-driven temperature sensitivity (Q10 coefficient) to atmospheric warming and is indeed retarded by other direct climate change effects, such as decreased soil moisture, and duration and thickness of snow cover.

(2) Indirect climate change effects, mediated through changes in plant

tissues, alpine vegetation communities and soil faunacommunities, can locally override the direct climaticcontrols.

(3) Physical litter breakdown may exceedbiological decay processes in unfavourable alpinehabitats and potentially benefit from stochastic extreme

climate events.

(4) decomposition in the Alpine environment tends to be internally buffered to observed changes in the climate. 

What are the developments that have taken place to study the rate of mineralization of litter ex-situ???

Litter decomposition in soils under high rainfall areas of the country like in Ultisols not only enriches OC status of soils but also enhances base saturation in surface horizons than the sub-surface areas. It is due to addition of alkaline and alkaline metal cations through the decomposition of litter fall. This way the progressive pedogenesis in soils of humid tropical climate is halted,suggesting why the occurrence of Oxisols are not reported from India in recent research results.

I agree to comments of Dr. Raj.

Please find enclosed an excellent work, worth reading...

Effects of Plant Functional Group Loss on Soil Microbial Community and Litter Decomposition in a Steppe Vegetation ( Front. Plant Sci. 8:2040. doi: 10.3389/fpls.2017.02040 )

Abstract: Globally, many terrestrial ecosystems are experiencing a rapid loss of biodiversity. Continued improvements in our understanding of interrelationships between plant diversity and soil microbes are critical to address the concern over the consequences of the decline in biodiversity on ecosystem functioning and services. By removing forbs, or grasses, or, to an extreme scenario, both forbs and grasses in a steppe vegetation in Inner Mongolia, we studied how plant functional group (PFG) loss affects soil microbial community composition using phospholipid fatty acid analysis (PLFA) and litter decomposition using a litter-bag method. PFG loss significantly decreased aboveand below-ground plant biomass, soil microbial biomass carbon (SMBC) and nitrogen (SMBN), but had no effect on the ratio of SMBC to SMBN. Although the ratio of fungal to bacterial PLFAs remained unaffected, PFG loss significantly reduced the amount of bacterial, fungal, and total PLFAs. PFG loss decreased litter monthly mass loss and decay constant, and such decrease was significant when both forbs and grasses were removed. Our results provide robust evidence that PFG loss in grassland ecosystem can lead to a rapid response of soil microbial activity which may affect litter decomposition and soil nutrient cycling, suggesting that the assessment of plant–microbe interactions in soils is an integral component of ecosystem response to biodiversity loss

Dr. Kumar! This is really very good information. Thank you.

Dr. Anoop Sir, I really appreciate you for raising such as important topic of litter decomposition. Litter decay just not depend on one climatic factors but it is been governed by several factors either individually or may be together. Decomposition rate varies with season, even with the variation in species. Temperate forest will have a different rate of decomposition then to tropical forest ecosystem.

Defining the criteria of rate of decomposition, C/ N and lignin components are the crucial factors defining the rate of decomposition in dry tropical deciduous forest. The functional leaf trait also plays and important role in governing decomposition ( The leaf with thick midrib and broader structure will undergo slow decomposition). Several other factors have already been discussed above.

Methods - Litter bad decomposition method is still considered the best method to study decomposition pattern

More information could be get from paper ..

Article Understanding the effectiveness of litter from tropical dry ...

Thank you sir for your nice question. Your questions always speed up our knowledge. However, you may go through this following links:

https://link.springer.com/content/pdf/10.1007/s11104-008-9658-5.pdf

Interesting explanation of mycorrhizal fungi by Dr. Raj. These fungi do good work during decomposition of litter or un-decomposed organic matter.

Yes, Dr. Raj! I agree with your answer.