In the short to medium term, it can only have been the literacy and comprehension tests. As explained below, the Poll Tax wasn't introduced until after the majority of black people in Louisiana had been disenfranchised.
In the longer term, it is impossible to say with any degree of certainty.
As Michael Perman observed in his book, Struggle for Mastery: Disfranchisement in the South, 1888-1908 [University of North Carolina Press, 2001]:
Of course, the exact cause of a person's failure to register was difficult, if not impossible, to determine since most of the unregistered had not been rejected when they applied but simply had not applied at all. The cause of their refusal or inability to apply cannot be ascertained.
In a 2018 briefing paper titled Barriers to Voting in Louisiana, the Louisiana Advisory Committee for the United States Commission on Civil Rights observed:
In Louisiana in 1896, there were 130,334 African Americans registered to vote; in 1900, after adopting a new constitution with aspects of the Mississippi Plan, there were only 5,320.
To understand this change, it is worth reviewing the changes implemented in the 1898 Constitution of the State of Louisiana with the provisions governing Suffrage and Elections in the 1879 Constitution of the State of Louisiana which it replaced.
Literacy and comprehension tests
That illiteracy was a significant factor is beyond doubt.
Prior to the adoption of the 1898 Louisiana Constitution, at the session of the Suffrage Committee held on 15 February 1898:
"... it was admitted that illiteracy was so great in Louisiana that an educational qualification would exclude too many whites, besides excluding the blacks"
However, Article 208 of the 1879 constitution had introduced a poll tax to fund public education:
Article 208. The General Assembly shall levy an annual poll tax, for the maintenance of public schools, upon every male inhabitant in the State over the age of twenty-one years, which shall never be less than one dollar nor exceed one dollar and a half per capita, and the General Assembly shall pass laws to enforce payment of said tax.
As Michael Perman noted in his 2001 book, already by 1898, many North Louisiana Democrats voiced their concern that
"... many African Americans would soon be able to meet the literacy test"
This was presumably - at least in part - due to improvements in public education funded by the poll tax introduced in 1879.
The literacy tests were introduced by Section 3 of Article 197 of the Constitution of 1898. As the figures from the Briefing Paper above attest, that was enough to have the desired effect of disenfranchising the vast majority of black voters (together with many poor, white voters).
However Louisiana legislators realised that that would not prevent black voters from regaining the franchise in the future. This was why North Louisiana Democrats lobbied for the poll tax.
The provisions introduced in Article 198 of the 1898 Constitution, requiring that all registered voters produced a receipt to prove they had paid the poll tax in the two proceeding years could not have been a factor in the dramatic decline in voter numbers noted above. It did not come into effect until after the general State election of 1890:
Article 198. No person less than sixty years of age shall be permitted to vote at any election in this State who shall not, in addition to the qualifications above prescribed, have paid on or before the 31st day of December, of each year, for the two years preceding the year in which he offers to vote, a poll tax of one dollar per annum, to be used exclusively in aid of the public schools of the parish in which such tax shall have been collected; which tax is hereby imposed on every male resident of this State between the age of twenty-one and sixty years. Poll taxes shall be a lien only upon assessed property, and no process shall issue to enforce the collection of the same except against assessed property.
Every person liable for such tax shall, before being allowed to vote, exhibit to the Commissioners of Election his poll tax receipts for two years, issued on the official form,or duplicates thereof, in the event of loss, or proof of payment of such poll taxes may be made by a certificate of the tax collector, which shall be sent to the Commissioners of the several voting precincts, showing a list of those who have paid said two years' poll taxes as above provided, and the dates of payment. It is hereby declared to be forgery, and punishable as such, for any tax collector or other person, to antedate, or alter, a poll tax receipt. Any person who shall pay the poll tax of another or advance him money for that purpose, in order to influence his vote, shall be guilty of bribery and punished accordingly. The provisions of this section as to the payment of poll taxes shall not apply to persons who are deaf and dumb, or blind, nor to persons under twenty-three years of age, who have paid all poll taxes assessed against them. This section shall not go into operation until after the general State election to be held in the year 1900, and the Legislature elected in the year 1908 shall have authority to repeal or modify the same.
While these restrictions may have proved useful later, when the population as a whole became more literate, by that point the vast majority of the black population had already been disenfranchised.
The question of the poll tax had divided the Constitutional Convention in New Orleans in 1898. As noted above, North Louisiana Democrats lobbied for the poll tax while those from the South of the state opposed it [Perman, Ibid, p142]. The opposition to the poll tax was largely on the grounds that it would also disenfranchise many poor, white voters.
In the end, those in favour of the proposal won the day and Article 198 was adopted, but with the proviso that it could be repealed after 1908.
Residency and record-keeping requirements
Most of the residency and record-keeping requirements adopted in the 1898 constitution were simply re-worded versions of those in the 1879 constitution. However, the additional requirements (for example those introduced in Article 197 of the 1898 Constitution) were not intended primarily to disenfranchise black voters, but rather to prevent the disenfranchisement of large numbers of white voters.
For example, section 5 of Article 197 states:
Section 5. No male person who was on January 1st, 1867, or at any date prior thereto, entitled to vote under the Constitution or statutes of any State of the United States, wherein he then resided, and no son or grandson of any such person not less than twenty-one years of age at the date of the adoption of this Constitution, and no male person of foreign birth, who was naturalized prior to the first day of January, 1898, shall be denied the right to register and vote in this State by reason of his failure to possess the educational or property qualifications prescribed by this Constitution; provided, he shall have resided in this State for five years next preceding the date at which he shall apply for registration, and shall have registered in accordance with the terms of this article prior to September 1, 1898, and no person shall be entitled to register under this section after said date.
This is the so-called Grandfather Clause.
The date chosen, January 1st, 1867, is significant. That was more than a year before the 14th Amendment to the United States Constitution was ratified (on 28 July 1868), granting citizenship to
"all persons born or naturalized in the United States"
which, of course, included former slaves who had been freed after the Civil War.
The exemptions to the educational or property qualifications set out in this section would thus apply almost exclusively to white men only.