Hi Cesar,

I´m not sure if you can use the C/N ratio as a one to one proxy for N-limitation of plants. Because also other parameter influence directly the nitrogen availability and sometimes P and not N is the limiting factor.

Additional you have to consider the soil typ, or the DIN concentrations, like ammonium and / or nitrate and .....

We have done some studies on N-cycle in Permafrost soils not in a global or rough scale. As reference literatur I have used:

Aber, J.D. (1992): Nitrogen cycling and nitrogen saturation in temperate forest

ecosystems.- Trends Ecol. & Evol. 7(7): 220-224.

Schimel J.P, Kielland K & Chapin F.S. III (1996): Nutrient availability and

uptake by tundra plants.- In: J.F. REYNOLDS & J.D. TENHUNEN (eds),

Landscape Function and Disturbance in Arctic Tundra, Springer, Berlin,

Heidelberg: 203-221.

Schimel J.P. & Bennett. J. (2004): Nitrogen mineralization: challenges of a

changing paradigm.- Ecology 85(3): 591-602.

Dear Tina, yes, I am aware that a perfect indicator of N availability does not exist, but as far as I know C:N seems to be the best indicator of N mineralization. So I acknowledge I am exposed to criticism because C:N is an imperfect proxy, but I want to give it a go for modelling purposes. 

In this case, I am exclusively interested in N availability, and not P, because I want to analyse the role of mycorrhizae under N limitations only, because these mycorrhizal effects are hypothetically different under P limitations.

Thanks

 Mr.Terrer,As I understand it is not the soil C:N ratio,it is the C:N ratio of freshly added manure ,crop residue or compost which decides the organic matter decomposition and mineralization.The soil C:N ratio normally reported is 10:1 to !2:1.The microbial bodies may have C:N ratio of 7:1.Very good quality composted manures may have C:N ratio of

I fully agree with the Dr. Rao observation that N immobilisation starts at C:N ratio above 30:1. In general N mineralisation is more between C:N ratio 10:1 to 15:1. The most appropriate depth to measure soil C:N ratio is 5-15cm of soil depth.

Yes, the C;N ratio between 20:1 and 30: 1 is  ideal. However, it may vary from soil to soil. Depth of sampling  is also very important as plant availability will depend on maximum root density zone as well.

C:N=25:1 approximates the cut off value. < 25:1 promotes mineralization and plant can grow without N limitation. >25:1 causes immobilization of inorganic N by decomposing microbial biomass.

DrsTarfdar,Mandal and Saha,I appreciate you comments.I have comeacross both the C:N ratios,25:1 and 30:1 in literature (even with in a standard  Soil Science book in two different chapters) as critical C:N ratios above which immobilization occurs.When legume straws are  applied to field, we hardly notice any immobilization of N or yellowing of the crop.This may support the C:N ratio of 30:1 as the critical limit.If there are any refined studies on this aspect ,I request the collegues to provide .

If C:N ratio is greater than 20 then immobilization generally takes place. Crop suffer from N deficiency in such situations. 

In the central United States maize crop belt they use a side dress Nitrogen application test to reduce the application of supplemental Nitrogen for maize and apply only when it is needed.  

When the maize crop is 15 cm tall approximately they sample the soil for the nitrate content.

When soil nitrate exceeds 25 ppm they do not apply supplemental Nitrogen for the maize crop. With this in mind for a crop like maize a high responder the threshold for economic Nitrogen an indicator of responsiveness is a level of less than 25 ppm Ntrate. 

The leaf analysis is also a helpful tool for fine tuning nutrient need.  Looking at the greeness of plants is also an effective indicator of N need which can be aided by spectrometer SPAD readings. 

Dear 

I agree with Dr Rao. The ideal C:N for manures of 20:1 If the soil has no organic manure/crop resisue there is no question of soil C:N ratio. 

the best C/N ratio is 1/15 increase in this ratio causes decrease in plant growth, for example in polluted soils with petrol compounds the C/N ratio is high  1/100 the number of microorganisms decrease 100-1000 times in comparing with non polluted soils ,and the plant growth is limited (very low). 

Dear all, most of you have suggested that the ideal C:N ratio for plant growth promoting N mineralization is around < 20-25. Please, bear in mind we are talking about global patterns, including forests, not crops in particular, and not the best amendments, just soil C:N globally. See the attached global map of C:N ratio from ISRIC-WISE. Boreal forests, that are consider N limited, have C:N ratios between 16 to 29. Tropical forests, that are considered P limited, with relatively high N, have C:N ratios between 10 to 15. Therefore, and simply based on available evidence, it seems that globally the C:N ratio that indicates N limitations should be around 15, whereas 20-25, as some indicated, is far too high and typical of extremely N limited areas. What do you think?

I appreciate the comments given by other researchers. As Dr. Rao indicated, the fresh plant materials and manures have high C/N ratio. If they are composted before application using appropriate composting procedure, the final mature compost is attributed by a lower C/N ratio of 15 – 20 and a higher pH value, compared to the starting feed mixture. It can contain considerable amount of plant available NO3-N. In general, the C/N ratio below 30 can be ideal for availability of plant nutrients and plant growth. In addition to the properties of the amendments applied, the biological and chemical properties of soils are very important in optimizing the C/N ratio.

C:N ratio of mineral soil is constant. The range is explained by many in the answers. Under normal conventional cultivation, it is not only the C:N ratio which is  going to have limitation of available N to plants, but also the total content of C/N going to decide it. Even with normal C:N ratio, if both the quantities are less and as both runs parallel (one increases with increase of the other and vice-versa), the available nitrogen will be limiting for plant growth .

Good comments by Dr.Agegnuhe.We have to distinguish three situations here-a native stabilized C:N ratio in soil as dictated by local climate.In that soil the usual C:N ratio could be 10-12:1 or slightly high ,even upto 15:1 provided it is continuouly receving inputs of fresh organic matter  every year or every season and decomposing it (and attaing the ratio of 15:I or less).This situation may also be similar to that of forest.Ideally decomposed manures may attain a C:N ratio of 15: to 20:I.That manure may contain humified organic matter and some quantity of already released nutrients.When such manure is added to soil further mineralization can occur until C:N reach a stable value of 12:1.The third situation is when organic material such as undecomposed/ partially decomposed animal manure, green manure crop or legume residue with C:N ratio of 25-30  is added to soil ,it undergoes decomposition,releases N  and attain ultimately  C:N ratio of  15:1 or 12:1,depending on continuity of carbon inputs.When manure /crop residues inputs are not regular, the chances for reaching a lower ratio of 12:1 are more than 15:1. In agricultural soils N starvation may not occur if C:N ratio is in the range of 16-29:1 as N mineralization can occur in that soil under such condition.

Thanks Dr. Rao, your optimal values of 10-15 C:N ratio seem to correspond with the C:N map attached (i.e. boreal forest N limited, tropical areas high N availability). Please, could you provide some references where it is stated that C:N ratios higher than ~15 usually limit plant growth due to low N availability?

Considering that the atmosphere is over three quarters Nitrogen with effective biological Nitrogen fixation Nitrogen should not be a key limiting factor in our environment or food system. It certainly can but our systems are not designed based around that key ingredient. 

In fact as Cesar brings out for the tropical world with the most potential to remediate our ovely rich atmosphere in greenhouse gases Phosphorus and I would say pH are the key limiting factors.

At the same time most of the Phosphorus needed for this key limitation is pissed and pooped away. In many ways we are like the standed raft ........water water it is every where yet not a glass of it nothing to drink. 

On another level Phosphorus can be limiting but without water that is moot point. For our water relation Carbon is the key and that is indicator that floats the nutrient boat including water and minerals. 

Food for thought. 

Dr. Hepperly,your  comments are very valid.As per current information,90 per cent of Indian soils with low to medium P status(

The majority of mineral agricultural soils ha a C:N ration 10-12. Any C:N ratio above this value means a significant amount of organic material input suffering from decomposition due to limited nitrogen or phosphorus in terms of chemical composition or environmental factors limiting the composition such as water scarcity, salinity or any toxic chemical afecying microbial community.

Dear Dr. Uygur, your response is exactly what I have observed for global patterns. But please, could you cite any reference to justify a cutoff between 12 to 15 C:N?

Thanks

When I looked into the map the C:N ratio increases towards pools due to very low temperature dependent restiction in organic matter decomposition. In the dry regions the organic matter added stay longer without decomposition which increase the c:N ration. In the tropics there are plenty of organic matter addition and suitable decomposition environment but excessive presipitation leache away the available nitrogen and therefor some increse occur in the C.N ratio. In this respect, If you overlap the temperature and rainfall map on the C:N map you can figure it out what I already said. On the other hand you can make a search on web of sci by using C:N ratio, nitrogen mineralisation, and organic amendment key words. You can then extract the information from the outcaming references (about 31 articles). 

An understanding of soil C:N ratio is necessary, which is one of the soil quality indicators. Knowledge of crop C:N ratios are also important to select crop types to be grown and keep a cropping system on the right track toward sustainability. The C:N ratio of 25:1 may  support soil microorganisms. If crops with high C:N ratios are grown frequently in the cropping sequence, residues will accumulate on the soil surface nitrogen may be scarce for crop growth unless supplemented with other sources of nitrogen. This may result in poor crop performance during times when soil microorganisms fix nitrogen while working to decompose high C:N ratio crop residues.

According to literatures, the dividing line between immobilization and release of N is about 20:1. A C:N ratio of  >30:1 results in immobilization during the initial composting process, while C:N ratio of  20-30:1, neither immobilization or release of mineral N, and   25:1 will result in a temporary deficit in N (immobilization), and those with a C:N ratio less than 25:1 will result in a temporary surplus in N (mineralization). Thus, soils with high C:N ratios can  “immobilize” N in the soil, because of the need of the micro-organisms for nitrogen. But once the ratio drops below 25, then further organic matter decomposition produces mineralized N,  i.e. surplus to the microbial requirements. Awareness of carbon to nitrogen ratios of organic materials applied to the soil is also important to manage soil, crop nutrient cycling and optimize availability of plant nutrients.

I think the what C:N ratio for the availability of plant nutrients is not the issue in this discussion, but how to optimize and maintain a suitable C:N ratio range that sustain both soil quality and plant growth in a certain cropping system.  The different soil management practices followed matter to maintain an ideal soil quality for optimum nutrient availability and plant growth.  

Dr.Agegnuhe,I appriciate your good comments.When we talk of C:N ratios and their role in N mineraliztion, we have to remember 4-5 points.1.In forest systems which are some what closed systems the C:N ratios will stabilize at some level(12-15:1 or even more) depending on climate,soil and continuous carbon input through leaf fall or leaf litter received on soil 2.Agricultural systems are many times disturbed systems(conservation agriculture may differ).They receive crop residues and root residues regularly.But manure and green manure inputs  are crop dependent or climate and irrigation dependent-both manure and green manures can be applied in irrigated system and green manures in irrigated system like rice and sugarcane.3.Most of our C:N ratios and mineralization studies are carried out in laboratories and green houses and we are extrapolating to field situations.Under field conditions manure or green manures are applied well before sowing of crops and incorporated in to soil.So a C:N ratio of 25:1 Or 30:1 probably does not matter as the initial N requirement  for seedling is small .4.If regular inputs of carbon/organic matter are small, the C:N ratios could be near 10-12:1 .5.In irrigated systems with 2-3 crops per year, there will be continuous inputs of organic matter through crop residues,roots and manure/green manure. In those conditions the C:N ration can be in the range of 12-15:1 as continuos inputs of organic matter continuous season after season.There is a need for generating C:N ratios in different climates,soils,cropping systems/forest types and managements under stabilized agriculture.

Yes, Dr.  Rao you have made excellent points. "There is a need for generating C:N ratios in different climates,soils,cropping systems/forest types and managements under stabilized agriculture." Understanding of the  inreatction  between climate, soil, water, microbes and crops is very important that affect soil quality and crop growth.

Dr. Rao compliments to you for brilliant comments

C:N ratio of agricultural lands is more or less constant. Except, where additional residues or organic matter are added. We all know the C:N at which mineralization will take place. Not only the C:N ratio, but the amount of organic matter which decides the amount of N in soil will decide plant nitrogen availability. For better nitrogen nutrition we need to maintain higher organic matter, which in turn dictated by climate and vegetation. 

For highyields we need to supplement N from external sources.  

Dr. Subba Rao has provided excellent comments on C:N ratio of organic matter and the management of OM in different forest and agro-ecosystems. We need to study more the need for and management of N in organic farming systems where there is a need to keep the residues on the surface for longer time (as long as possible) to protect the soil from erosion and to conserve soil moisture, while the decomposition of some organic matter will release N for crop uptake. The twin objectives of keeping the residues on soil surface that requires organic materials with wider C:N ratio (C:N 30:1 or higher) and the decomposition of organic matter for releasing N to crops that requires a narrow C:N ratio (C:N 12-15:1). 

Dr.Balasubramanian,Thank you for your comments.As mentioned by you mulching provides good opportunity for recycling of wide C:N organic materials and get benefits you have highlighted.

Dr Getachew , Dr Malhotra , Dr Mahapatra and other colleagues, C:N ration in most of agriculture soils remains more or less constant . If you have an nitrogen exhaustive crops or cropping sequence , dont you think , C:N ratio being a constant value , it will fluctuate more or less equally with respect to both, c as well as N , quite unlike forest soils  ? 

Yes, Dr. Anoop, I agree. But if additional information is available in both N exhaustive crops and cropping systems in relation to C:N ratio that will be useful to substantiate this conception.

Please, you are all going off topic except Dr. Rao. My question is intended to be focused on global patterns of soil C:N ratio in vegetated areas. Please, stop focusing on crops only, crops are just a small fraction, with natural vegetated areas as the largest by far. 

We all have seen on my map that C:N ratio for 0-20cm depth goes up to 20-25 C:N maximum in boreal areas, that are strongly nitrogen limited. Therefore, it is clear from the map that there should be a threshold of optimum C:N probably around 12-15 CN (see new map attached). The thresholds you are all proposing of around 20-25 are clearly not valid for the assessment of nitrogen availability for the global estimations of C:N ratio at 0-20cm. 20-25 C:N is far too wide to represent optimal N availability on this map.

Please, I just need you to help me find references to justify a cutoff set at 12-15 C:N, which seems to fit the observations. Thanks

Any organic material with C:N ratio below 20:1 will release N for crops. See:

http://www.extension.umn.edu/agriculture/tillage/soil-management/soil-management-series/organic-matter-management/

Mr.Terrer,While going through an article I have come across a citaion of the following paper which gives a C:N ratio of 14.3+-0.5 for  overall soils and 8.6+-0.3 for soil microbial biomass .For more details you may refer to the publication.Cleveland,C.C .and Liptzin,D.2007 C:N:P stochiometry in soil:is there a "Redfield ratio for the microbial biomass? Biogeochemistry 85(3):235-252.

Mr.Terrer, I think,  that this article will be useful for you:

srep35496 (2016)

Sincerely, Zenon

C:N ratio and N availablity will not only depend on residue nature, weather condition play an important role. We have seen C:N ratio >20 immobilize N in rice field. It might be different with other vegetation.

For those interested in this question, I recommend reading: C. Wang et al., Aridity threshold in controlling ecosystem nitrogen cycling in arid and semi-arid grasslands. Nature Communications. 5, 4799–4799 (2013) 

Wang et al. found a threshold of 0.32 in aridity index (ratio of precipitation to mean temperature) below which plant N uptake is limited by water availability, and characterised by low soil C:N despite extremely low soil N content. Above this threshold, water limitations on plant N uptake stop, and N accumulation occurs.

Dr.Cesar,good information.

I have a query regarding the C:N ration . Is it possible that the % of TOC is lesser than the % of TKN??  As I am doing the analysis for desert soil (Soil is mostly Sandy in nature (up to 70-80%) with very less or no vegetation. TOC was analysed by Walkley black method and Total nitrogen by Total Kjeldhal Nitrogen method. 

Dr. Singh, in exceptional cases it is possible. In Thor Desert of Jaisalmer where only sand dunes are there (95-99% of sand) and in Camel grazing areas on that sand dunes we are also experiencing similar observation. But these are exceptional cases, although practically it is possible.

Then where  this nitrogen is coming from and in what form , it is persisting into the soil ..let it be any kind of soil..?

Dear Tarafdar Sir,

I am also analyzing the Thar desert sample near Sam area, Jaisalmer. So, can i use the data in my work (what justification should i give) or is there any other method or protocol that i can use to calculate the C:N ratio. 

The N is coming probably from the urine and camel excreta as large number of camel are gossiping there for the tourist.

Does it stay fro so long under such high temperatuue and evaporative demand..

Sir, most of the population near the Jaisalmer area live their life as a cattle farmer. They own hundreds of sheep and cows. So i guess what Dr. Tarafdar is saying is right.

You can download the Total N dataset from WISE30sec and compute the average value

http://data.isric.org/geonetwork/srv/eng/catalog.search#/metadata/dc7b283a-8f19-45e1-aaed-e9bd515119bc

~0.2%

Under Indian conditions, predominantly tropical,the soil organic carbon may range from from 0.2 to 0.8 per cent and total nitrogen may range from 0.02 to 0.08 per cent.It will be low in sandy or light textured soils and high in clay or heavy textured soils.Also irrigated rice soils contain more carbon than the dry lands.Soils under forest and grasses may contain more carbon than under arable/ cultivated soils .Long term cropping and balanced fertilization and also manuring builds up both C and N in soil .Integration of both fertilizers and manures may have better effects on soil carbon and nitrogen dynamics.

Dear Desika, I think 1g/m2 = 0.01%

Let us look at the problem from the point of organic matter addition (differing C:N ratio) to soil. As an organic matter sources having a C:N ratio above 30 net immobilization takes place. Between 15-30 both mineralization and immobilization are to occur. In this case assume typical harvest residue of wheat is 3000 kg ha-1 with 80 C:N ratio (40%C and 0.5%N). Typical C:N ratio of soil is 10-12 with 1% organic carbon content. After incorporation of straw in the 0-20 cm layer usually a net immobilisation is to occur.

In this case the total C will be 2,500,000 kg soil ha-1 x 0.01 + 3000 x 0.4= 26200 kg C ha-1

total N =250000 x 0.01/12 +3000 x 0.005 (N content of straw) =2098.3

the C:N ratio is then 26200/2098.3 = 12.486

this may mean a C:N ratio above 12 can limit mineralization of incorporated organic material having high C:N ratio. On the other hand the total mineral nitrogen content (NH4 and NO3) of soil at any organic matter incorporation has a crucial significance. If the available or mineral nitrogen can able to reduce C:N ratio of the added organic matter below 30 after incorporation then mineralization and immobilization simultaneously occur, a C:N ratio below 15 means a net mineralization.

Dear desica,

The convertion of % into kg/ha and therefore g/m2 can be done as follow.

%N x 10000 x d x Bd x 10 =kg/ha then

(kg/ha)/0.1= g/m2

where 10000 is convertion factor of % into mg/kg, d is soil depth (m), Bd is bulk density (g/cm3 or tonnes/m3)of your soil.

Dear Singh

I would like to give an example of high nitrogen in desert soils. Chilean Nitrate (NaNO3), also known as Natural Nitrate of Soda (NNS), is a mined product from a desert in Northern Chile, which is the only known deposit of this mineral salt in the world. In a similar maner, you may have similar type deposites with much lower tenor. As you know rain water consists of some nitrogen in NH4 and NO3 forms. This type of nitrogen in low-land may be accumulated in time. Then the nitrogen content gradually increase whereas no C increment.

Sir,

I have a doubt,

C/N ratio < 10, what is its meaning

C is organic carbon or total carbon?

Cesar Terrer have you got the answer? I am also interested in C:N ratio. For norther forest understory, can I say C:N ratio represents productivity? is there any threshold value for unproductive vs productive soil? My study area is located in a semi-arid zone. thanks.

Dear Arsha Krishnan, C/N< 10 can be expressed as Carbon to Nitrogen ratio is less than 10. Ratios of Carbon to Nitrogen less than 10 are quite Nitrogen rich and Carbon is not so high that Nitrogen availability will be curtailed. Hope this is useful for your understanding. High C/N ratio in soil is associated with reduced Nitrogen availability.

I follow answers

best regards

Hey guys, I also got C:N ratios of 8 and 9 in vertisols and apedal soils, respectively and I wanted to believe that this indicates high OM decomposition rate and mineralisation. However, due to high waterlogging in vertisols relative to well aerated apedal soils, I do not expect soil fauna to be more active under low oxygen conditions. Can any one help with some explanations.

While nitrogen was not limiting oxygen was. Vertisols respond to raised bed culture the raised bed allows for more conclusive drainage aiding in aeration.

The raised bed used in the dry season can be supplemented with drip irrigation and row cover. The healthy start can make it possible to have better responses because susceptibility to anoxic environment is maximum in seedling stages and can become less later.

You will want to select crops and varieties that adapt well to vertisols.

Thanks Paul for the answer, actually I am investigating the effect of these different soil types on vegetation including richness, composition, diversity and some physiological responses of woody species on conditions of these soils. As you say, very few woody plant species (n = 6) were adapted to vertisols compared to apedal soils (n = 14).

can anyone tell me what does a C:N ratio of 288:1 represent?

is it possible to have this large value for C?

the sample used is a sandy soil agricultural soil

Mr. Rizwana , the sandy soil mentioned by you appears to be very poor in nitrogen .In agricultural soils it is very rare to come across such N poor sandy soils, though the sandy soils are generally low in both C and N. What is the crop / crops grown in such soil? What is the amount of dry matter produced on such soil?. What is the general weather in the area. What are the actual amounts of C and N in the soil studied?

The nitrogen content is very low (0.1064g/kg). Soil is found along a coast. I am assessing leaching of nutrients on these soil. The climate there is very hot and humid. There is cultivation of onions on the fields. Results from my experiment shows that the loss on ignition % is 5.26.

Mr Rizwana,

By considering the Kjeldah N result your soil shoud have an organic matter content around 0.19-0.22%. Beacause there is nearly constant ratio between soil C and N (C/N) around 10-15 depending on the difering climatic zone. Probably you have same artifact in your results. Loss on ignition leads some weight loss from soil carbonates and sulfates even some criystalline water in the minerals. What I suggest is change your analysis method and work with a CRM.

Veli Uygur Okay i will change the method used and thank you :)

Can anyone tell me the available amount of phosphorus found in sandy soil?

i need some research data to compare with my results

thank you

@Khadaroo, in general it is between 3 and 15 ppm.

How to obtain 3% nitrogen from urea. thanks to explain this in (gm for nitrogen) for adding to l litter water

3% Nitrogein as gram for i liter water =30gram per liter water since urea contains 46% nitrogen it means dissolving 30* 100/46 = 65.217 g/l or dissolving 65.217 gram of urea in one liter of water gives one liter of solution contains 3% of nitrogen.

many thanks prof. Akram

That is weight base or volüme base 3%. If it ise volüme base 65.217 g urea should be dissolved in water by making 1 L of final volüme. If ıt is on weight base 65.217 g urea should be dissolved in 934.783 g of water.

In my opinion, the so-called sufficient or insufficient nitrogen in soil, limited or not, depends on three aspects: 1) the absolute amount of available nitrogen in soil particles per unit volume at a certain point in time, 2) the rate of nitrogen absorption per unit volume by root system, 3) the rate of soil nitrogen mineralization.